Dietary chlarella has known as one of the best candidates for development of multifunctional probiotic foods owing to an excellent nutritional value such as high amount of proteins and various, valuable fatty acids. So many efforts were devoted to studying the chlorella as therapeutic agents or foods fighting against many diseases in the aged people such as cardiovascular diseases and cancers. In this study, we investigated sizes and weights of tumors derived from mice injected subcutaneously with tumorigenic cells to see if antitumor activity would be found in mice dieted with the chlarella complex. After BALB/c mice were dieted with $5\%$ organic cultured chlorella complex diet throughout for 19weeks, the fibrosarcoma was induced by subcutaneous injection of tumorigenic cells at the 3 weeks before sacrifice. The average weight of tumors in the diet group were significantly reduced to $60\%;(P=0.012)$ of the one in control group, indicating that diet with the chlarella complex may have anticancer activity in mice. When the mice were dieted with $5\%$ organic cultured chlorella complex for 4 weeks before injecting the tumorigenic cells in order to see tumor-preventive effect of the diet, the potential preventive activity of the diet against cancer was implicated by the observation that the tumors were greatly reduced in the diet group to $37\%$ (P=0.l44) of the control group. Especially, when the $5\%$ diet were applied to mice after injecting with the tumorigenic cells, the tumors derived from the $5\%$ diet group were also decreased to $95\%$ (P=0.002) of those in the control group, suggesting that the diet with the organic cultured chlorella complex may also have therapeutic effect against tumor formation. As results, it was shown that the chlorella complex tested in this study had preventive and therapeutic effects on fighting against tumorigenesis. Therefore, the identification and further mechanistic study of the components which may be associated with antitumor activity from diet of the chlorella complex in the future will contribute to the development of anticancer probiotic foods, alternative therapeutic treatment against cancer, and a new anticancer drug.