A protease, actinidin, was isolated from Cheju kiwi fruit Actinidia chinesis. The enzyme was purified about 8.5 fold with the yield of 25% by column chromatographies of DEAE-Toyopearl and Sphadex. G-100. Purified enzyme gave a single protein band on polyacrylamide gel electrophoresis and its molecular weight estimated by SDS-PAGE was about 27, 000. The optimum pH and temperature were 7.0 and 4$0^{circ}C$, respectively. This enzyme was stable at the ranges of pH 5.0~9.0 and below 5$0^{circ}C$. It was also found that Fe+2, Fe+3, and Na+ ions increased enzyme activity, whereas Hg+2 and Co+2 ions decreased. The enzyme was inhibited by phenylmercuric acetate and leupeptin, which indicated that active center of the emzyme had thiol-group. The enzyme reaction followed the Michaelis-Men-ten dkinetics with the Km value of 0.32 mM for casein.