FCC gasoline is a major component in the total gasoline pool produced in an integrated refinery, but it contains many compounds (olefins, sulfur and aromatics) which lead to harmful automobile emissions. The objective of the present study is to determine the effect of feedstock quality on gasoline composition in a range of operating variables with a constant type of catalyst. The work was carried out in an FCC pilot plant constructed and operated in CPERI. The FCC gasoline was fully analyzed in a system of GC/MS. Ten different feedstocks were used in the unit in order to investigate the feedstock physical properties which affect the gasoline yield and composition, the feed conversion and the coke yield as well. The gasoline components were measured as total hydrocarbon groups: aromatics, normal and branched olefins, normal and isoparaffins and naphthenes but special emphasis was given, in this study, for the aromatic and olefinic content of gasoline. The main conclusion of the work is that feed conversion, coke yield and gasoline yield and composition are strongly influenced by the type of FCC feedstock. It was shown that a paraffinic and an aromatic FCC feedstock produce, respectively, an olefinic or an aromatic gasoline. The hydrotreating process plays also an important role in the gasoline composition. For these feed effects detailed qualitative and quantitative information is given in the paper. Moreover, short form models were proposed for the prediction of conversion coke yield and gasoline composition as a function of the main feedstock properties. Analytical forms of these models are presented for gasoline aromatics and olefins and total conversion as well. The predictions of the models were satisfactory for all hydrocarbon groups. The models were also validated with experiments using two additional feedstocks in the pilot unit under a wide range of experimental conditions.