Green tea intake is known to have preventive effects against cancer. In this study, we evaluated the tumor suppressive effects of dietary green tea extracts (GTE) as a modulator on cisplatin in an established colon cancer mouse model. The cisplatin-induced cytotoxicity was determined with cell viability of the mouse colon cancer cell line (CT26) in vitro. The influence of GTE on the anti-tumor activity of cisplatin was evaluated by measuring tumor size with digital calipers in mice bearing CT26 colon carcinomas. The CT26 cell viability decreased to 93% at a $20{mu}g/mL$ concentration of cisplatin. However, cell viability decreased to 15% with a combination of $20{mu}g/mL$ cisplatin and GTE ($75{mu}g/mL$). There were no apparent changes in cisplatin-induced cytotoxicity with GTE and epigallocathechin gallate (EGCG) treatments. Tumor size decreased in dietary GTE combining intra-peritoneal cisplatin-injected tumors bearing mice compared with cisplatin or GTE alone administered to tumor-bearing mice. These experiments showed that dietary GTE has a potentiating effect on the cisplatin anti-tumor activity of an established mice colon cancer model. Therefore, the GTE may be a candidate for modulators in anticancer treatments with cisplatin.