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The Lengguru Basin is one of the several frontier basins of Eastern Indonesia. Although a number of wells have been drilled there since the late 1980s, petroleum exploration in this basin is still immature. The occurrence of hydrocarbon seeps in the southern area revealed that an active source rock is present in the Lengguru Basin. However, source rock characteristizaion in the Lengguru Basin is poorly constrained, especially for the Mesozoic sections. This paper focuses on the organic geochemical characterization of each of the the Mesozoic formations, based on twenty-one outcrop samples. The primary objective of this assessment is to evaluate the source rocks potential of the Mesozoic interval. The Mesozoic formations of Kopai, Piniya, Ekmai, and Imskin, present each a relatively high organic richness, with a respective Total Organic Carbon (TOC)’s content up to 1.61%, 1.35%, 17.65% and 1.93%. Most samples can even be classified with good-excellent organic content. Several samples appear to show characteristics of gas prone Type III kerogens and appear to have generated hydrocarbon. Based on the results of this assessment, the petroleum system in this area was divided into two super groups which are Kembelangan (Kopai, Woniwogi, Ekmai and Piniya Formation) and New Guinea Limestone Groups (dominated by Imskin Formation). The high maturity could have been attained by the heat which was produced during the period of intense thrusting and uplift between Miocene and Pliocene. Moreover, samples were collected in the eastern area which is metamorphic zone, causing the samples to have higher maturity than other areas in this basin. Despite of the maturity, petroleum exploration in this basin is still favorable which is indicated by high organic content and hydrocarbon seeps. Further study and exploration should go to southwest area and subsurface of Lengguru Basin to find economic reserves. Keywords: Lengguru Basin, Mesozoic, Source Rock