The cave site at Khao Ta Phlai is a small limestone outcrop, located at Rap Ro Sub-district, Tha Sae District, Chumphon Province. It is accessible to the Gulf of Thailand via the land and waterway routes around 20 km at Chumphon Province, and around 35 km to the Andaman Sea at Ranong Province. The early maritime ports have been found in the two provinces. The idea of trans-peninsular routes on the peninsula is raised, but no data to be proved. Therefore, the excavation at Khao Ta Phlai would be a promising site to give the insight into the issue. The systematic excavation, classification of finds, samples for Thermoluminescence dating and interpretation of the data were undertaken. The result demonstrates that the site has three periods: the first occupation dating back to 3842±376 BP; the second 2629±257 BP; the third 2477±242 BP and 2031± 199 BP. The first and the second occupations were hunter-gathers, confirmed by stone tools and polished adzes in associated with animal bones. By 650 years ago, after the second occupation of the lithic hunter-gathers, new dwellers occupied the cave (2235 – 2230 BP); they were involved in communication with people of the coastal areas, confirmed by imported materials such as glass beads and bronze objects from the early ports of both coasts: Phu Khao Thong (c. 2100 – 2000 BP) and Pak Chan site (c. 2000 BP) in Ranong Province on the western coast; Khao Sam Kaeo (c. 2 500-2000 BP) and Khao Sek site (c. 2200) in Chumphon Province on the eastern coast. These suggest that the cave dwellers communicated with people outside their community. These findings strongly confirm the significant role of the cave dwellers at Khao Ta Phlai, the hinterland site, involved in trans-peninsular routes that facilitated the early ports of two coastal areas on the Upper Thai Peninsula.