The study was designed to observe the effect of different dietary fats on the incidence of colorectal tumor and in vivo cell proliferation in colon carcinogenesis. Male Sprague Dawley rats were intrarectally infused with chemical carcinogen(methylnitrosourea, MNU) and fed 16%(w/w) fat diet containing one of dietary fats(beef tallow, corn oil, perilla oil) for 30 weeks. To measure in vivo cell proliferation, the incorporation of 5-bromo-2-deoxyuridine(BrdU) into DNA was localized using the monoclonal anti-BrdU antibody. Large number of tumors were found in the distal colon and tumor incidence was increased in the order of perilla oil(57.7%)<corn oil(82.1%)<beef tallow diet(88.5%). Cell proliferation of colonic mucosa in beef tallow fed rat was significantly increased to give longer crypt length compared to those of corn oil and perilla oil fed rats, which resulted in higher incidence of colon tumor. Corn oil feeding did not significantly increase cell proliferation but gave higher incidence of tumor compared to perilla oil. Thus, n-3 $alpha$-linolenic acid rich in perilla oil could have a protective effect against colon cancer compared to saturated fatty acid or n-6 linoleic acid.