By bus[edit]
Buses operate every 20 minutes or so from Bangkok's Northern Bus Terminal (Moh Chit*) directly to Ayutthaya (despite what locals say, the last bus leaves at 18:00). First-class air-con buses charge 56 baht. This trip is scheduled to be around an hour and a half, but allow at least two hours for the trip since the buses stop rather frequently and there are often jams on the roads out of/into Bangkok.
To get to the Northern Bus Terminal, go to Mo Chit BTS Station. Upon exiting the gate, cross the bridge on the right to go to the bus stop, and take bus service 3 or bus service 77. (air-con buses charge 12 baht, non air-con buses charge 7 baht.) Bus ride is about 10-15 minutes and the Northern Bus Terminal destination is the last stop for the bus services. However, buses do not stop in the Northern Bus Terminal, but at the bus stop across the street. Cross the bridge to get to the bus terminal.
Bus service 3 runs also near Khao San. It goes by Chakrabongse Rd which is a street on the western end of Khao San. The trip to the Northern Bus Terminal from here takes around 1 hour. Getting back in the evening can take longer due to traffic. Official metered taxi directly from Mo Chit bus terminal taxi stand to Khao San Road costs 105 baht.
Also you can take a minivan from the Victory Monument direct to Ayutthaya. Takes ~1 hour and costs 60 baht. Minivans depart every 20 minutes or so.
The buses run from 04:30–19:15. For more details, call Tel. +66 29 362 852-66 or see the website [2] and Ayutthaya Bus Terminal, Tel. +66 35 335 304.
In Ayutthaya, the central BKS bus station is on the south side of Naresuan Rd next to the Chao Phrom Market. songthaews to Bang Pa-In also leave from here. Some 1st-class buses to Bangkok, however, leave from the north side of the road some 500 m to the west, on the other side of the khlong (canal); the queue for air-con buses is easy to spot.
From Kanchanaburi, take a local bus of horrible quality from the main bus station to Suphanburi for 50 baht (2.5 hours + long waiting for bus and departing) and then official airconditioned "minibus" (actually Toyota Hilux minivan) nr. 703 to Ayutthaya for 80 baht (1 hour). Big bus from Suphanburi to Ayutthaya was cancelled and replaced by minivan service nr. 703 even according timetable on bus station, although there is no price mentioned on timetable so 80 baht is probably farang (foreigner) price and locals pay less (they use this minivan service too). The best advice is avoid this route completely, because it will cost you 130 baht and you will tired after spending 4.5-5 hours (incl. waiting) to get from Kanchanaburi to Ayuthaya, so it's actually faster go back to Bangkok and get to Ayuthaya from Bangkok without any cheating or double prices and more comfortable for same amount of time or even faster and only little bit more expensive. A taxi from Kanchanaburi costs 2,000-2,500 baht (2 hours).
There is also a central bus station east of town serving northern destinations. It can be reached by songthaew. Ask around to find the appropriate stop.