ABSTRACT: Diatom occurrence, abundance, and biostratigraphic position are documented from subsurface Albian - Maastrichtian succession penetrated by the Raad-1Well, drilled at the north-eastern Sinai, Egypt. The examined stratigraphic section is differentiated into four lithostartigraphic units from base to top: Kharita Formation (Albian), Halal Formation (Cenomanian), Wata Formation (Turonian) and Sudr Formation (Santonian to Maastrichtian).Well tomoderately-preserved marine diatom assemblage included 87 species and varieties representing 37 genera have been identified. The assemblage is mainly dominated by planktonic taxa rather than periphytic forms. Most of the encountered diatom species have long stratigraphic ranges, except 39 taxa are belonged to Cretaceous period, and taxonomically enumerated. Two diatom biozones are recognized based on the distribution and stratigraphic occurrence of the Cretaceous diatoms in the studied section. Amblypyrgus campanellus Zone is characteristic the lower part of the succession (Kharita Formation), and point to Lower Cretaceous. Gladius antiques Zone is encountered from Halal,Wata and the lower part of Sudr Formations, and denotes the Upper Cretaceous age. The Albian / Cenomanian boundary is placed at the last occurrence ofAlbian diatom taxa and the first appearance of Late Cretaceous diatom forms. The microfloral content and lithological characters of the examined section reflect a shallow marine environment prevailed during the Early Cretaceous, which changed to be more deeply environment in the Late Cretaceous.