In order to investigate the relationship between the methylphenanthrene index（MPI1）and the vitrinite reflectance（Ro），the extracted chemical composition of marine shale that dominated by type Ⅱ kerogen and the aromatics fraction in its related crude oil in the Upper Cretaceous of a basin in Western and Central Africa Rift Systems were analyzed using gas chromatography-mass spectrometry（GC-MS）technology. According to the distribution patterns of phenanthrene and methylphenanthrene and other polycyclic aromatic hydrocarbons，the relationship between the methylphenanthrene index and the vitrinite reflectance of the source rocks in this area was researched，and the maturity of source rocks and the related oils in this basin were evaluated. It shows that the Ro value has an excellent positive linear correlation with MPI1 ranging from 0.50% to 0.90% with a coefficient of 0.92. This equation is significantly different from that proposed in previous literature based on coal and type Ⅲ kerogen. The difference of Ro value between the immature to low mature stage and high mature stage after oil window can approximately reach 0.2%. Therefore，it can cause significant effect on thermal maturity of the organic matter in the source rocks and hydrocarbon resources assessment. The oil maturity was evaluated by this equation. The results are in good agreement with the geological background of this basin.