As precursor fiber of advanced SiC fiber, cured polycarbosilane (PCS) fiber is prepared, and thermostatic pyrolysis of this fiber is studied in detail. Since the weight change is the most important characteristic of the pyrolysis degree of cured PCS fiber, a precise balance is applied on-line to follow the weight change of the cured PCS fiber, which was carried out in a standing style furnace. In thermostatic pyrolysis, the degree of pyrolysis, which is characterized by the weight loss of the fiber, and the properties of the final SiC fibers were found to be strongly dependent on the process conditions such as N2 flow and the amount of fibers. From much evidence, it is the offgas evacuated in the process that plays an important role by accelerating pyrolysis and increasing pyrolysis degree.