The effects of trace carbon element on microstructure and mechanical properties of a near beta titanium alloy Ti-4Al-5Mo-8V-2. 5Cr-1Sn-2Zr through various heat treatment were studied. The results show that the addition of carbon element has a significant effect on the microstructure after different heat treatments. The addition of carbon leads to the refinement of beta grains. And after the solution treatment,the size of beta grains decrease from 137 μm to 25 μm and becomes more uniform with the increasing of carbon volume fraction from 0% to 3%. And beta grains begin to grow as the carbon content continues to increase. After aging treatment,the size and volume fraction of secondary alpha phase decreases with the increasing of carbon. Compared with the samples after single-aging treatment,the samples after double-aging treatment present the more secondary alpha phase. Additionally,with the increase of carbon,grain boundary alpha phase reduces gradually through single-aging treatment. Due to the influence of grain refinement,Ti C particles and secondary alpha phase,with the increase of carbon,the micro-hardness and tensile strength of the alloy increased significantly,but the ductility decreases.