This study was carried out to investigate the effects of B4C or Y2O3 additives on the tendency of sintering, $eta$-SiC synthesis and mineral phase changes by reaction bonding of SiC at 145$0^{circ}C$. At the sintering temperature of 145$0^{circ}C$, the additives such as B4C or Y2O3 did not improved porosity and bending strength. Added more than 1.5% of Y2O3, 0.5-0.3% of B4C, the formation of $eta$-SiC was increased. At higher temperature above 145$0^{circ}C$, it seems that the bodies added B4C, contained 3C form of SiC were denser than that of Y2O3 added. Because the transition of 3Clongrightarrow4Hlongrightarrow6H promoted sintering.