An analytical model for the isothermal crystallization of fiber reinforced polymers is presented. The model is based on approximate expressions for the volume of intersection between a sphere and cylinder. These expressions are used to account for the effect of the fibers on the overall crystallization process, Expressions for the average volume of spherulites truncated by the fibers are computed. The crystallization process is divided into time frames during which specific types of fiber truncations are encountered. Three different time sequences for the occurrence of the truncations are also derived according to the fiber volume fraction. The depressing effect of the fibers on the overall crystallization process is demonstrated with simple examples.