Abstract: The fracture mirror method was modified and applied to evaluation of the in-situ strength of fibers in ceramic composites. The most significant modifications include (1) the mirror constant relies on the tensile test of fiber bundles instead of a single fiber test and (2) the gauge length parameter is redefined. Traditionally, the gauge length parameter was considered as mean fiber pull out length. In this study, by analyzing the mechanism of fiber pullout, the parameter was defined as the sum of double pullout length and double ineffective length of the broken fibers and the random parameter was then graded statistically. Based on this method, Weibull parameters and mean in-situ strength of the M40JB fiber embedded in SiC matrix composites produced from polymer precursor were obtained. The experimental results show that the Weibull shape parameters of fabricated fibers and in-situ ones are nearly identical, but the scale parameter of in-situ fiber strength is much lower than that of the fabricated one. That means the flaws position distribution in both fibers does not vary obviously, however, the flaws are deeper in in-situ fiber than in fabricated one and that leads the strength of the former fiber to decrease markedly.