Technical challenges in using diatoms for paleolimnological work are the identification and enumeration of diatom valves. Variations exist in the minimum number of valves to identify, ranging from 100 to 700 valves of the dominant species. This task can be very time consuming, particularly when the diatom valves are not abundant. This research was conducted to determine the minimum number of valves to be identified in the diatom assemblages from Rawapening Lake, Central Java, Indonesia. Based on the 314 samples obtained from Rawapening Lake, the diatom efficiency rose above 0.85 upon the minimum count of 300 valves. The number of diatom species identified remained stable after the minimum of 300 valves. Therefore, the minimum number of diatom's valves identified to represent the assemblage for paleolimnological analysis was 300.