Polyisobutenylsuccinic anhydride(PIBSA), an intermediate for the lubricating oil additive, was prepared by the reaction of polyisobutylene(PIB) with maleic anhydride (MA). The functionality, which indicates the extent of reaction of PIB-a and MA, was determined in the various reaction conditions : fuctionality was 0.98 under the reaction conditions of no solvent for 12 hours at 190℃, 0.21 in benzyl alcohol solvent for 12 hours at 190℃, and 0.03∼0.20 with various Lewis acids such as AlC1₃, SnCl₄, Et₂AlCl, and TiC1₄. The fuctionality also depended on the structure of PIBs. As α-olefin content (exo-form) in PIB increased, the fuctionality had a higher value. The structure of PIBSA prepared from PIB and MA was determined with FT IR and ¹H NMR spectroscopy. Two strong anhydride IR bands at 1782 and 1855 cm^-1 were obserbed and two IR bands at 1639 and 897 cm^-1 for unsaturated groups of PIB disappeared. The presence of the anhydride was difficult to find by ¹H NMR spectroscopy because the anhydride protons gave relatively small peaks over a 2.0∼3.0 range. Polyisobutenylsccinimide (PIBSI), a lublicating oil additive, was prepared by the reaction of PIBSA with diaminoethane.