Prachinburi is about 135 kilometres east of Bangkok — roughly 2.5 hours by car or bus, depending on traffic.
By road: Highway 304 is the most direct route from Bangkok, heading northeast through Chachoengsao and into the provincial capital. The road is well-maintained and fast outside peak hours. If you are driving yourself, Highway 304 also gives direct access to the national park zone further east: continue past Prachinburi town toward Kabin Buri to reach the fringes of the Dong Phayayen–Khao Yai Forest Complex.
By bus: Buses depart regularly from Bangkok's Ekkamai (Eastern Bus Terminal) and Mo Chit (Northern Bus Terminal) to Prachinburi. Journey time is 2.5 to 3 hours. Minivans also run from Ratchadaphisek area and are faster but less comfortable.
By train: Prachinburi has a railway station on the eastern line. Several trains a day run from Hua Lamphong station in Bangkok, with journey times of 3.5 to 4.5 hours on the ordinary trains. The line continues east toward Sa Kaeo. Train travel is slower than the bus but gives a more relaxed journey through the central plains.
From Khao Yai (Pak Chong): If you have already visited Khao Yai from the northern side, you can continue through the park and exit via the Prachinburi side — though check that the through-road is open, as it is sometimes restricted. Otherwise, backtrack to Pak Chong and take Route 304 south and east.