Abstract: In recent years, the great breakthroughs have been made in gas exploration in East Africa. A series of large gas fields have been discovered in the offshore of Tanzania basin and Ruvuma basin, with a total recoverable reserves of 38 000×10⁸ m³. In order to understand the geochemical genesis of gas fields, we studied the structural evolution characteristics of the basin. The basin has gone through three stages of tectonic evolution, which includes the Karoo rift, Madagascar rift and Madagascar drift. The Lower-Middle Jurassic restricted sea high-quality source rocks formed in the Madagascar rift period and the hydrocarbon generation and expulsion stages are from late Early Cretaceous to Oligocene. In this paper, the gas composition and carbon isotope characteristics in offshore of Tanzania basin are analyzed in detail, the gas maturity is discussed, and the gas genetic type is identified. The results show that the gas is oil-formed gas with high maturity, and the value of gas maturity Ro is between 1.2% and 2.6%, which is in line with the LowerMiddle Jurassic marine high-quality source rocks. Therefore, it is presumed that the gas originated from the Lower-Middle Jurassic marine high-quality source rocks.