The influence of heat treatment temperature from 1400 to 2840 degrees C and time from 1.2 to 12.0 min on the structure and mechanical properties of polyacrylonitrile carbon fibers was studied. It was observed that the Young's modulus increased with increasing temperature and time, but the tensile strength exhibited different variation trends with the different processing methods. For a fixed time of 1.2 min, the strength dropped from 4.6 GPa at 1400 degrees C to 2.6 GPa at 2840 degrees C, (similar to 43.5 %) as opposed to a 63.9 % increase in Young's modulus. However, when the treatment time was increased to 6.0 min at 2500 degrees C, the tensile strength decreased only by 1.9 %, from 3.71 to 3.64 GPa, versus a nearly 20.0 % increase in Young's modulus. The same situation was found for treatment at 2000 and 2700 degrees C. Raman spectroscopy and uniform stress model analysis show that the degree of covalent cross-linking between the graphene planes decreased as temperature increased, while it remained almost constant as treatment time was increased. It is believed that during heat treatment of a carbon fiber, the cross-linking collapses at the beginning but the crystalline size keeps growing with prolonging time, so the tensile strength decreases little with further heat treatment while tensile modulus keeps increasing.