A large number of site-specific restriction endonucleases have been isolated from a wide variety of microorganisms because of their extensive usefulness in the structural analysis of DNA molecules, and for the construction of recombinant DNA in vitro. Through this thesis, I present a procedure of purification of restriction endonuclease AccI from Acinetobacter calcoaceticus ATCC 23055. I also describe the determination of physical and catalytic properties of this enzyme. For the purification of the enzyme, 300 g cells of wet weight were used and were broken by French press at 20,000 p.s.i.. After ammonium sulfate fractionation, the enzyme was further purified by heparin agarose column chromatography, DEAE-sephadex A-50 column chromatography, Affi - Gel Blue column chromatography, phosphocellulose column chromatography, and finally hydroxyl apatite column chromatography. The isolated enzyme has shown to have site-specific restriction endonuclease activity on the AccI recognition sequence which has a degeneracy i.e., : d (5``-G-T-A-T-A-C-3``) or d(5``-G-T-C-G-A-C-3``). AccI endonuclease has a single polypeptide species and its molecular weight of subunit was $45,000 \pm 1,000$ daltons, as judged by 10\% polyacrylamide gel electrophoresis in the presence of 0.1\% sodium dodecyl sulfate. In order to study the catalytic properties of AccI endonuclease, I was used (3H)-labeled DNA by HpaII methylase as a substrate. The enzyme showed maximum activity at pH values between 8.0 and 11.0, and in the presence of 5.0 to 20.0 mM magnesium chloride. But AccI endonuclease did not require salt (NaCl) and 2-mercaptoethanol for its activity. Furthermore the enzyme retained its maximum activity to the concentration of 100 mM sodium chloride and to the concentration of 300 mM 2-mercaptoethanol.