Abstract: In the preparation of fiber reinforced ceramic matrix composites (FRCMCs) by polymer-infiltration-pyrolysis (PIP), the volume of the composites would change during the conversion of the preceramic polymer into a ceramic matrix in the first pyrolysis process. The repetition cycle-density densification model is revised by introducing the volume shrinkage percentage of the composites. The densification models for the FRCMCs preparation by active filler and/or inert filler-containing preceramic polymer-infiltration-pyrolysis are also developed. The relationships between the theoretical density and porosity of the composites and the repetition cycle are discovered. For the active filler-containing system, the theoretical density not only relates with the ceramic yield of the polymer and the density ratio between the pyrolysis product and polymer, but also relates with the reaction yield and density ratio of the active filler after reaction, and the volume shrinkage of the composite. It is proved that the addition of the active filler is more effective than that of the inert filler for the composite densification.