Objective: To investigate to the effect of vitamin C (VitC) on enhancing the sensitivity of breast cancer cells to paclitaxel (PTX) chemotherapy and the mechanism. Methods: In primary cultured human breast cancer cells after exposure to VitC alone or PTX alone or their combination, the proliferation, apoptosis and expressions of caspase-3 and Bcl-2MTS were determined by MTS assay, flow cytometry and Western blot analysis, respectively. Results: In breast cancer cells, the IC50 of VitC alone was 2.5 mmol/L and PTX alone was 8.6 nmol/L, while IC50 of PTX combined with 1 mmol/L VitC was 2.8 nmol/L. VitC alone had certain apoptosis inducing effect on breast cancer cells, and the apoptosis inducing effect of PTX combined with VitC was significantly greater than that of PTX alone, which was also increased with increase of the concentration of VitC (all P<0.05). The caspase-3 expression was increased and Bcl-2 expression was decreased significantly after exposure of PTX alone, and these effects were enhanced by combination with VitC, and increased with the increase of the concentration of VitC added (all P<0.05). Conclusion: VitC can enhance the sensitivity of breast cancer cells to PTX chemotherapy, and this action may be related to its further increase caspase-3 and decrease Bcl-2 expressions.