Ostracod faunas from the Upper Cretaceous, Palaeocene and Eocene of Northwestern Libya (Well B1-NC41, Well C1-NC41, Well K1-NC41, Well Bla-137, Well D2-NC41, WeU H1-NC41, Well J1-NC41, Well N1-NC41, Well F1-NC41 and Well L1-NC41) are described in detail. One hundred and thirty-three ostracod species and subspecies belonging to fifty-five genera and subgenera are described in the present study. Of these seventy- seven species are new, twenty-three species are related to taxa previously described from adjacent areas. Two species are regarded as having affinities with species described in other areas and thirty-one species and four new genera are left under open nomenclature, mainly because pf lack of material or the impossibility of observing the internal features. Five ostracod biostratigraphical zones are proposed for the Tellil Group (Harsha, Dahman and Samdun Formations); four zones belong to the Middle Eocene and one zone is established for the Upper Eocene. The ostracod zones have been studied in each Well of the study area. The age of these formations has been determined by the ostracod zones. The ostracod faunas from the Middle and Upper Eocene of the Ghalil Formation, the Palaeocene and the Maastrichtian of the A1 Jurf Formation, from Late Palaeocene of the lower part of the Farwah Group and the Maastrichtian of Bu Isa Formation have been studied in detail. Their age is also determined by the ostracod species. The affinities of the Libyan Ostracoda genera and species to those from other parts of the world, largely North and West Africa are discussed. The environment of deposition of the formations studied has been determined by the using the characteristic species of ostracods and other fauna and the type of the sediments.