Abstract: To explore the potential mechanism of the MCF-7 human breast carcinoma mouse model. Methods: Electron paramagnetic resonance ( EPR ) was used to measure the PO2 level directly. Tumor tissues were collected and their mitochondrial activity was assayed using enzyme kinetics; the expression of NADH-DH, CcO, and SCR proteins were also detected by Western blot analysis. Results: The data from tissue oximetry demonstrated that tumor tissue was hypoxic compared with normal breast tissue ( 10.3 ± 0.4 ) mmHg vs. ( 14.1 ± 0.3 ) mmHg, P < 0.05, n = 6 . Mitochondrial NADH-DH, CcO, and SCR function in the tumor tissue were lower than those in normal breast tissue at ( 177.16 ± 2.01 ) nmol of reduced NADH/( mg protein·min ) vs. ( 137.59 ± 3.73 ) nmol reduced NADH/( mg protein·min ), ( 14.81 ± 0.89 ) vs. (19.53 ± 0.35 ) nmol cyt c reduced/( mg protein·min ) and ( 103.96 ± 5.38) vs. ( 124.78 ± 1.34 ) nmol cyt c oxidized/( mg protein·min ), respectively ( P < 0.05 ). The expression of NADH-DH, CcO, and SCR proteins in the tumor tissue was lower than that in normal tissue. Conclusion: Detection of PO2 level in breast carcinoma in mouse model with EPR directly proves that solid tumors have a hypoxic microenvironment. The complex enzyme activities were all decreased, which maybe related with the decrease in protein expression.