Cancer chemopreventive agents are designed to reduce the incidence of tumorigenesis by intervening at one or more stages of carcinogenesis. This study aimed to determine the effects of resveratrol (RES) and tannic acid (TA), which are chemopreventive agents, on the nitric oxide synthase (NOS) levels that are effective for development of cancer in colon and breast cancer cell lines. The CaCo-2 (human colon carcinoma cell line) and MCF-7 (Michigan Cancer Foundation-7; human breast adenocarcinoma cell line) cells were grown in the laboratory. RES and TA were used to treat CaCo-2 and MCF-7 cells. Nitric Oxide Synthase Assay Kit was used to determine the NOS enzyme activity of CaCo-2 and MCF-7. Statistical differences between control and RES-and TA-treated cells were calculated using the Student's t-test for double comparison. It was observed that NO activity was generally decreased in CaCo-2 and MCF-7 cells, in which RES and TA were applied. Results suggest that the phenolic compounds RES and TA have different effects on NOS enzyme activity of the colon and breast cancer cells.