Abstract: The old-growth, multiple ages, multispecies natural forest has played an important role in terrestrial ecosystem dynamics model and the global carbon budget. However, carbon fluxes of old forests in subtropical regions are rarely reported in China. In the present study, the CO₂ flux of an old-growth subtropical evergreen and deciduous broadleaved mixed forest was observed using -eddy- covariance technique in Tianmu Mountain of Zhejiang Province. Based on the data sets which were observed from July 2013 to June 2014, the variations of net ecosystem exchange (NEE), ecosystem respiration (R_e), and gross ecosystem exchange (GEE) were analyzed. The results showed that during the study period, the monthly NEE all had a negative value (acted as a carbon sink) except for December and February (acted as a carbon source). The average monthly NEE was -61.52 g C·m^-2, the monthly carbon sequestration showed a doublepeak curve and the maximum carbon sink was -149.40 g C·m^-2, which occurred in June, while the maximum carbon source was 23.45 g C·m^-2, which occurred in February. The maximum of monthly mean CO₂ flux occurred in June with a value of -0.98 mg·m^-2·s^-1, while the minimum value occurred in December with a value of -0.35 mg·m^-2·s^-1. The NEE at the time point of positive and negative conversion had typical seasonal characteristics. The yearly NEE, R_e, and GEE were -738.18, 931.05 and -1669.23 g C·m^-2, respectively. Compared with other forest ecosystems located at the similar latitude, the carbon fixation of the old-growth forest was larger, likely due to its complicated structure within the canopy and the presence of young-growth regeneration and successional stands. This showed that other than in carbon neutral, old-growth forests of Tianmu Mountain in subtropical China had a strong capability in carbon sequestration.