Abstract Planktonic foraminifera assemblages were studied for biostratigraphy and paleoclimatic interpretation of the Upper Oligocene-Lower Miocene of Temburong Formation, Klias Peninsula, Sabah. The Temburong Formation is composed of mainly flysh-type argillaceous deposit intercalations of slightly calcareous pelagic shale. Recently we discovered abundance of moderate to well-preserved planktonic foraminifera at the measured section and its planktonic assemblages are useful for biostratigraphy and paleoclimatic studies of Temburong Formation. A total of nine samples of shale were collected from three outcrops and processed based on standard micropaleontology method. Twenty-eight species of planktonic foraminifera were identified and grouped into three zones, namely: P21-P22 Globorotalia opima opima-Globigerina angulisuturalis zone, N4 Globigerinoides primordius-Globigerinoides quadrilobatus zone, and N5-N6 Catapsydrax dissimilis-Globigerinoides altiapetura zone. For paleoclimate studies, the relative abundance of planktonic foraminifera was obtained and grouped into climatic indices namely cool, cool-temperate, warm, warm-temperate and none climatic diagnostic. The result shows the changes of planktonic foraminifera assemblages and the paleoclimatic indices through Oligocene-Miocene boundary. The cool climate indices were identified at the lower part of the study section of Upper Oligocene sediment and gradually shifted to warm climatic indices at Lower Miocene section.