Travelers can take a flight, bus or train to Siem Reap. Getting to Cambodia from Thailand by bus is the cheapest way. If you’re not on the tightest budget, get the direct bus, not the package bus. Tickets range between US$17 – $32 (or 600 – 1100 baht). The border crossing is long though. Usually, the bus is safe but isn’t too comfortable. When crossing the border from Thailand into Cambodia, you have to be more cautious in Poipet (border crossing scam).
It’s also a good idea to get the Cambodian visa online rather than at the border crossing. However, if you’re taking a direct bus from Siem Reap to Bangkok and vice versa, Nattakan Transportation‘s buses are one of the two choices you’ve got. From Bangkok to Siem Reap, Nattakan bus usually departs Morchit Bus Station at 8.00 AM and reaches Poipet border (Thailand-Cambodia) at around 12:30 AM. Visa on arrival takes you about 10 minutes. By 4:30 pm on the same day, you’d arrive Siem Reap. On the bus, the bus attendant provides you breakfast, coffee, juice, and lunch. Nattakan bus drops off the passengers on Sivatha Road, Krong Siem Reap, the central of the city.
Nattakan Cambodia started in 2012. On December 14, 2012, the cross-border bus company entered an agreement with Thai National Scheduled Bus Company to upgrade bus services between Cambodia and Thailand. This has made it really easier for travelers to take direct between Siem Reap and Bangkok.
You can easily book your bus tickets online through: BookMeBus.com
Mekong Express is also one of the favorite buses among expatriates.
Updated: Starting in December 2016, Giant Ibis also add this route between Bangkok (Thailand and Siem Reap (Cambodia) to its list of departures and destinations.
Web link: Traveling Round Trip by Bus from Bangkok to Siem Reap