Abstarct: The Kazhdumi Formation, due to its diversity of lithology and its function as a source, reservoir and cap rock is one of the most important formations in south-southwest of Iran, the Persian Gulf and the neighboring Arab countries. This study compares the organic geochemical properties of the Kazhdumi Formation in the Persian Gulf and Zagros fold-thrust belt to investigate the significant role of the formation within hydrocarbon systems of the region. In this study, the sedimentary basin analysis has been considered to examine the organic geochemical properties of the Kazhdumi Formation within a frxamework of the Neo-Tethys Basin. Two subsurface intervals of the formation in two oil fields at the northwestern Persian Gulf and one outcrop succession of the formation at the Nang-e-Maghar, Zagros Folded Belt have been investigated. For this purpose, Rack-Eval analyses, X-Ray diffraction, petrography studies by field emission secondary electron microscope (SEM) and point elemental analyses (EDS) have been done. The rock-Eval analyses demonstrate that the kerogens in the Zagros are mostly type II and slightly type I, however in the Persian Gulf are generally type III and partly type II. baxsed on this, the organic matters associated to the kerogen type III, which are sourced from the terrestrial plant near the coast, were deposited in the shallow marine part of the Neo-Tethys, at the Persian Gulf. Conversely, the organic matters related to the kerogen type II, were concentrated in the deep marine part of the basin. The petrographic studies of the surface samples from the Zagros, by the SEM, displayed nano-fossils of planktons related to the deep sea environment. The results of analyses show that the Kazhdumi Formation has oil generation potential in the Zagros, but in the Persian Gulf, it could generate mostly gas and partly oil. The organic facies of the Kazhdumi Formation are in the range of D and CD in the Zagros, but are in variable range of B, BC, C, and CD, in the Persian Gulf. The variation of organic facies in both studied area indicate changes of the sedimentary environment and fluctuation of relative sea level during at the time of sedimentation. The geochemical studies also show that in spite of high TOC content of the Kazhdumi Formation in the Tang-e-Maghar section (3.7 %wt.), the formation is at the immature stage. However, the TOC of 1.6 to 2.6 %wt., and maturity of the Kazhdumi Formation in the Persian Gulf reveled the hydrocarbon generation in this area. The XRD analyses demonstrate that the main clay mineral in the Kazhdumi Formation at both studied area, is the kaolinite, which is a clue for warm and humid climate during the lower Cretaceous.