Abstract: Detailed diatom analysis has been carried out on one hundred forty-two samples collected from the Lower to Middle Miocene stratigraphic succession located in the Abu Qada basin, west-central Sinai. The succession is composed only of two lithostratigraphic units belonging to Gharandal Group; they are Rudies Formation followed directly by Kareem Formation. The diatom analysis shows interesting diverse and abundant different diatom assemblages of both marine and non-marine origin.Atotal of 84 diatom species and varieties belonging to 32 genera were identified. The preservation of the recorded taxa varies from poorly preserved in some samples to generally moderately to well preserve in other samples. Several intervals of pronounced diatom dissolution detected throughout the studied section. A biostratigraphic zonal framework consisting of three planktonic diatom zones is proposed and the Early /Middle Miocene boundary is determined accurately by means of diatom taxa.