C/SiC composites,were prepared via isothermal chemical vapor infiltration (ICVI), A novel process of ill situ whisker growing and matrix filling during ICVI was devised to reduce the porosity of the C/SiC composites, by alternating the dilute-gas species. C/SiC composites with increased density were prepared successfully using this novel process, in comparison with those obtained from the conventional ICVI process. The whiskers seem to have grown into the large pores and modified the pore structure that is filled by the SIC matrix.