BACKGROUND Colorectal cancer is the third leading to death type of cancer in the world. The therapeutic guideline varied between different methods. As the main therapeutic guideline is chemotherapy, recent studies had shown utilization of natural products in combination with conventional medication, elevate the efficiency of chemotherapeutic methods. Kombucha is a traditional beverage obtained from the fermentation of green tea as a rich source of flavonoid medicinal plant. This study aimed to evaluate the natural potential of combination therapy of this natural product with doxorubicin as a chemotherapeutic agent. METHODS The study was performed as in vitro evaluation of biological activity of kombucha on HCT-116 cell line (human colon cancer cell line). The cytotoxic effect of different kombucha beverages (fermented green tea) in comparison with green tea extract was evaluated by dimethylthiazolyl tetrazolium bromide (MTT) assay. In the next step, anticancer activity of doxorubicin as a general guideline chemotherapeutic agent in combination with kombucha was evaluated by cell cycle analysis and apoptosis assay flow cytometry. Apoptotic genes expression pattern was determined using real-time polymerase chain reaction. The experiments were designed in three independent replications and statistically analyzed using SPSS software. RESULTS The results show that kombucha compared with the green tea extract caused more (1.2 fold) early apoptosis induction and G0/G1 phase arrest. Moreover, kombucha increased the expression levels of p21, p53, and B-cell leukemia/lymphoma 2 (Bcl-2)-associated X protein genes (2, 2.5, and 1.5 fold, respectively) while it decreased Bcl-2 gene expression level (5-8 fold) compared with doxorubicin alone. Combination of kombucha with doxorubicin shows 2-fold increased G0/G1 phase compared with the doxorubicin treatment. CONCLUSIONS This result indicated that kombucha caused boosted anticancer activity of doxorubicin agent. These findings suggest that kombucha may be has an assistor and useful role in colorectal cancer treatment align with chemotherapy.