The geological age of the outcrop along the Prataan River were interpreted based on calcareous Nannofossil and Foraminiferal assemblages. The lithological unit recognized in this area, from oldest to youngest are Ngrayong-Bulu, Wonocolo, Ledok and Mundu Formations. In this study, 23 samples of 15 sample locality were analyzed; 23 species of calcareous Nannofossils and 20 species of planktic Foraminifera were identified. Based on calcareous Nannofossil, the Ngrayong-Bulu, Wonocolo, Ledok-Mundu Formations are NN7 or middle Miocene, NN10 or late Miocene, NN14-NN15 or early Pliocene in age, respectively. Based on planktic Foraminifera, the Wonocolo is N16 or late Miocene, and Ledok-Mundu Formations is N18-N19 or Pliocene in age. Correlating the section's composite column of Prataan River with well TPN-1, revealing that the middle-late Miocene unconformity and the late Miocene-early Pliocene unconformity discovered in the well are predicted to be existed in the Prataan River outcrops.