While the temple is located in Cambodia, access to the temple is possible from Thailand as well, with no Cambodian visa required.
From Cambodia
A packed laterite access road from Siem Reap via Anlong Veng, a distance of over 200 km, was completed in 2003. The road from Siem Reap to Anlong Veng has recently been fully paved, and most of the road from Anlong Veng to Preah Vihear has been paved, with an approximately 20 km stretch remaining a dirt road but in the process of being paved. While a 4x4 is not necessary to make it to the base of the hill on which Preah Vihear is located, a 4x4 or moto will be required to scale the steep road going up the hill.
Dancing Roads (http://www.dancingroads.com) regularly arranges multi-day bike trips from Phnom Penh to Preah Vihear.
You can also reach the place on a three day motorbike trip from Kompong Thom.
As of June 2010, Preah Vihear could only be accessed by going through a Cambodian military base. While the temple can be accessed by tourists, you should bring both a native Cambodian guide and small denominations of Cambodian Riel for "gifts" to soldiers and their families. In addition, while there is no official entrance fee, to scale the road leading to the temple, you may be required to give gifts to military closing off the road.As of March 2012, Preah Vihear could only be accessed by bying ticket for 5 USD by tickets office downhill. For this you get motorbike with driver, who brings you uphill to temple for couple of hours. No gifts required for soldiers.
From Thailand
NOTE: Due to a lingering border dispute, Thailand periodically blocks access to the site from its side: as of February 2009, the park including Moh I-Daeng cliff are open, but it is not possible to cross the border. Inquire locally before travelling.
As of June 2010, the Thai-Cambodia border was closed off completely with razor wire, and thus Preah Vihear is unable to be accessed from Thailand.
The nearest significant Thai town is Ubon Ratchathani. The temple is at the end of Route 221, but public transport options are limited and the easiest option is to charter a car for the day 1000 baht and up, plus gas. The roads are surprisingly good and, depending on how hard your driver hits the gas pedal and/or how many water buffaloes decide to cross the road along the way, you can get there from Ubon in an hour and a half.
If this is out of your budget, the nearest town of any size is Kantharalak, which can be accessed by frequent public bus in 2 hours or so from the nearby towns of Ubon Ratchathani and Si Saket. For the last leg of the trip 34 km, however, you will have to hitchhike or charter a songthaew/tuk-tuk/moto taxi.
At the entry gate into Khao Phra Wiharn National Park, you will have to pay a 200 baht entry fee Thais 20 baht; note that the park is open only from 8 AM to 3:30 PM. The road ends at a large parking lot, the final leg less than a kilometre into Cambodian territory you will have to cover on foot. At the Thai immigration post you'll be charged an additional 5 baht for a second ticket, and you'll also have to show your passport - they'll take a photocopy, but no stamps are issued and no visas are needed. After the road ends, walk over the smooth rock surface to the entry gate and pay another 200 baht fee this one to enter Cambodia and get your ticket punched, and now you can proceed to the ruins.
How to get there: From Bangkok, use highway 1 Pahol Yothin Rd. turn right at Saraburi into highway 2 Mitraphap Rd.. At Amphoe Si Khew, turn right into highway 24, and travel via Amphoe Pak Thongchai, Sangka, and Ku Khan. Turn right into highway 221, and head to Amphoe Kantaralak and keep going to the park.
From Ubon Ratchathani, use highway 2178 and 221 via Amphoe Varin Chamrap, Samrong, Benjalak, and Kantaralak to the park.