Some 130 km. south of Phetchabun on Highway 21. and another 9 km. alter a left turn, lie the ruins of Si Thep.

An ancient town where many structures still remain indicative of its past prosperity during the 11th-18th Buddhist Centuries, Si Thep was the centre of contact between the Dvaravati Kingdom in the Central Plains and the Khmer Kingdom in the Northeast. There are widespread remains of laterite buildings and of several ponds. Artefacts found here include elaborate lintels, Sema stones (used to mark out the limits of Buddhist temples) and jewellery.