Effect of SiC particle size of the densification of Al2O3-SiC composite during pressureless sintering was investigated. Two types of SiC powders having average particle size of 0.15${mu}{ extrm}{m}$ and 3${mu}{ extrm}{m}$ were used. Densification rate of the specimen containing 0.15${mu}{ extrm}{m}$ SiC particles was slower than that of the specimen containg 3${mu}{ extrm}{m}$ SiC particles. Although the relative density of the specimen containing 0.15${mu}{ extrm}{m}$ SiC particles was below 90% of theoretical density after sintering at 155$0^{circ}C$ the complete closure of open pores occurred. Therefore full densification could be obtained by subsequent HIP. On the other hand in the specimen containing 3${mu}{ extrm}{m}$ SiC particles the complete closed pore was observed at 95% of theoretical density. Such a fast pore closure in the specimen containing 0.15${mu}{ extrm}{m}$ SiC particles is likely to occur as a result of dense reaction layer formation on the specimen surface which is attributed to the high reactivity of small size particles with sintering atmosphere.