There are two international airports in Bangkok: Don Mueang International Airport(DMK), Suvarnabhumi International Airport(BKK).
Don Mueang International Airport(DMK), which in the north of Bangkok, about 25km to the downtown, mainly acts as domestic airport with a few international airlines.
Suvarnabhumi International Airport(BKK), which is in the east of Bangkok, about 35km to the downtown, acts as international airport. Most international airlines are in this airport.



From Suvarnabhumi International Airport(BKK) to Downtown:

1)The most convenient way is by taxi, about 20mins to downtown.

2)The most comfortable way is by Airport Rail Link.

I. SA City Line: takes 30mins to Phaya Thai Station, about 45 THB.

II. Airport Express Line: takes 15mins to Makkasan Station directly, about 90 THB.

III. New Airport Express Line(non-stop): takes 18mins to Phaya Thai Station directly, about 90 THB.

Rapid Transit Systems in Bangkok

Bangkok is currently served by three rapid transit systems: the BTS Skytrain, the underground MRT and the elevated Airport Rail Link.

1)The BTS(Bangkok Transit System) consists of two lines, Sukhumvit and Silom, with thirty stations along 30.95 kilometers.
Skyline Silom Line: Bang Wa(S12) ←→ National Stadium(W1)
Skyline Sukhumvit Line: Samrong(E15) ←→ Mo Chit(N8)
*Siam Station is interchange station between Silom Line and Sukhumvit Line.

2)The MRT(Mass Rapid Transit) currently consists of two line, the Blue Line and Purple Line. Purple Line is under-construction.
MRT Blue Line: Hua Lamphong ←→ Bang Sue
*Si Lom Station and Sukkumvit Station are interchange station with BTS

3)The Airport Rail Link connects the city center to Suvarnabhumi Airport to the east. Its eight stations span a distance of 28 kilometers.
I. SA City Line: takes 30mins to Phaya Thai Station, about 45 THB.
II. Airport Express Line: takes 15mins to Makkasan Station directly, about 90 THB.
III. New Airport Express Line(non-stop): takes 18mins to Phaya Thai Station directly, about 90 THB
*Makkasan is interchange station with MRT blue line; Phaya Thai Station is is interchange station with BTS Silom Line.



For more details, please see http://www.bts.co.th/customer/en