A detailed microstructural analysis of slip-cast Si3N4/SiC-whisker composites has been made by TEM. In spite of the gentle forming process, SiC whisker breakage constitutes a fundamental feature of this material. The breaking takes place during the sintering process and could be associated with residual thermal stresses, as revealed by the experimental evidence. The dramatic decrease in the whiskers aspect ratio translates into little or no effect of whisker addition on the mechanical properties of the Si3N4 matrix.