From Bangkok to Siem Reap by Bus
From Khao San Road:Khao San Travel Mart (51 Trokrongmai, Chaofah RD., Pranakorn, Bangkok 10200) at 08:00, 09:00 and 01:00. The trip should take about 8 hours with border crossing. Usually, they pick you up at the Mc Donalds close to the police station at Khao San Road. But check your ticket before you miss the bus since this can change any time. You can book your ticket here: Khao San Travel Mart
From Morchit Bus Station to Siem Reap
Morchit Bus Station is also called Mo Chit or Chatuchak Terminal
Transport Co LA is a Thai government-operated company and has one bus from Morchit to Siem Reap at 09:00. The trip takes about 9 hours. Some days there is another one at 07:30 which usually is a bit faster.
Keep in Mind
You will cross an international border! You need a valid visa to do so. If you do not have a visa you can get a Visa on Arrival the border. It should cost you US$30 and they only accept clean dollar bills. If you want to pay in Thai Bath be prepared to pay more. Bring two passport pictures as well. Since May 2019 they usually scan your passport picture but if the machine is broken you better be prepared. Your passport needs at last one empty page for the sticker and some space for two stamps. Sometimes there is a mysterious separate 100 baht fee on top of the US$30 fee for the stamps. If the line is not too long you can insist to get your visa without tea money.
The bus company might arrange a special VIP service for another 5US$ to speed things up. Usually, you can do everything on your own save this money.
Most companies change the bus at the border but will bring your bags to the other side. Sometimes you have to bring them yourself