Calcareous nannofossil has been used for several decades as a biostratigraphic tool. The smear slide preparation is commonly used to study the calcareous nannofossils because it requires an easy, relatively quick, and small amount of sediment needed. However, to compare the preservation of the calcareous nannofossils, various optical mounting media were used in this study. This research was performed deliberately to address the quality of the medium used to maintain its longevity. The efficiency of the mounting media takes optimum consideration of the strong resolution under the transmitted light. Three different mounting media were used: a) nail polish, b) Canada balsam, and c) Norland Optical Adhesive. All the samples were prepared from the same site of Temburong Formation (Klias Peninsula, Sabah). These samples have been observed under a Leica ICC50E polarizing light microscope with x1000 magnification. Both cross-polarized (XPL) and plane-polarized images were used to compare the overview of taxa specimen’s visibility. Overall, Norland Optical Adhesive dominated the higher-resolution biostratigraphy potential as it was fast dried out, permanent, non-toxic, and less bubble formation. The image of the specimen was clear than the image using media of Canada balsam and nail polish.