I will arrive in bangkok airport at 6:30 in the morning and I want to take a bus to the cambodian border (Arunyaprathet) as soon as possible.
someone knows if there is a good bus directly from the airport to arunyaprathet and where I can find timetable ?


Can you tell me in which countries there are direct buses from capital city airports to other countries?
Not one in Bangkok either.
Get to the Mo Chit (Northern) bus terminal and hop on a bus to Aranya Prathet from there.
With a 0630 ETA, you'll have no worries getting to Siem Reap by 2000 same day.
As long as you're not planning to do this around Chinese New Year (around 18th Feb) or Songkran (mid-April), you'll be fine just making your way to Mo Chit and jumping on next available bus.
Have a great trip!