This question was posed at Lonely Planet‘s Thorn Tree travel forum back on June 20, 2011: Flight from Chiang Mai to Sukhothai?
If you refer to that thread, it seems that no such flight may exist. At least, no direct flights.
Referring back to a thread begun February 22, 2010, at TripAdvisor gives similar results: Flight from Chiang Mai to Sukhothai?
That would appear corroborated at website Thailandee.com‘s page Flights to go from Chiang Mai to Sukhothai. All options there are to fly to Bangkok from Chiang Mai, and then have another flight booked from there for Sukhothai. Your guess is as good as mine as to how valid the timetable on that page is.
Yet note this chart:
Note the third flight listed — a 35-minute flight daily? That chart is from Chiangmai-Thai.com. Please refer to that page directly, for the chart may have been long amended by the time you are reading this post, which I am editing on November 12, 2012. Also, I suggest that you refer to this TripAdvisor thread dated August 27, 2012: Can you fly round trip Chiang Mai to Sukhothai?
Bangkok Air used to have a flight, but although the schedule can still be found on line, you can’t book the flight on the Bangkok Air site, so I assume it’s been discontinued?
Another website — Travel.MapsofWorld.com — mentions the following statement in their page Flights to Sukhothai:
Another flight departs from Chiang Mai international airport and arrives at Sukhothai airport.
However, there is no indication how current that webpage may be.
Also, notice what 1StopThailand.com has to say on their page Transportation to Sukhothai:
Sukhothai’s airport is part of the growing network of privately-operated runways owned by the boutique carrier Bangkok Airways. It’s small but wonderfully designed with the tropics in mind, and if you want to get to Sukhothai fast this is the best option.
Bangkok Airways runs daily flights from Bangkok, taking 70 minutes, as well as daily flights from Chiang Mai, taking less than an hour. The private airport is located 27kms from Sukhothai along Route 1195. Buses are waiting to ferry passengers into the city for a reasonable fee.
A website called GetToandFrom.com doesn’t even mention flying as an option on their page How to get from Chiang Mai to Sukhothai.
The airport is located in Amphur Sawankhalok and is about 30 km away from downtown. Flights operate daily between Sukhothai – Bangkok and Chiang Mai.
I am sorry that I could not be more definitive on this topic, but I hope I have given you at least a little help.
Here are a few travel-related references that you may possibly find entertaining or useful:
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Publish Date: 11/12/2012 0:32
Chiang Mai Best – Travel Northern Thailand … Loy Krathong (ลอยกระทง), sometimes spelled Loy Kratong, is a colorful festival held every year on the full moon of the twelfth month in the Thai Lunar Calendar. This Thai Festival is held all over the country, but there are particularly beautiful celebrations held in Chiang Mai, Ayutthaya and Sukhothai, where the celebrations take place over several days. Celebrations are now held throughout Thailand including Bangkok, …
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Publish Date: 11/07/2012 19:37
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Publish Date: 11/09/2012 7:21
They are: Bangkok; Ayutthaya; Chiang Mai; Sukhothai; Tak; Samut Songkhram and Suphan Buri province. What is a Krathong? Krathong refers to a lotus-shaped tiny boatl made from materials such as banana leaves, banana …
Publish Date: 10/25/2012 1:51
Thailand’s northern province of Chiang Mai ranks as the top destination for Thais wanting to travel during the upcoming cool season. According to the sur…
Publish Date: 11/09/2012 0:26
The affected routes include Bangkok-Chiang Mai and Bangkok-Chiang Rai during Nov. 27-29. Although not an official public holiday, Loy Kratong is one of the most popular and romantic of Thailand’s traditional festivals.
Publish Date: 11/08/2012 7:12
This Thai Festival is held all over the country, but there are particularly beautiful celebrations held in Chiang Mai, Ayutthaya and Sukhothai, where the celebrations take place over several days. Celebrations are now held …
Publish Date: 02/11/2012 0:00
DD8318 B737-4D7 VTBD-VTCC
Uploaded by Renato L d Santos on Oct 23, 2009
Nok Air DD8318 B737-4D7 fom Bangkok to Chiang Mai Thailand