The effects of free silicon on thermoelectric properties of reaction-bounded silicon carbide (RB-SiC) were studied. RB-SiC ceramic consists of some free silicon after reaction sintering. Some RB-SiC samples were reheated at high temperature in vacuum to adjust silicon content. The results showed that free silicon affects thermoelectric properties remarkably, especially for the thermoelectric power. A surprising phenomenon was found in SiC/Si composite consisting ~20wt% free-Si, i.e., thermoelectric power, figure of merit Z and ZT are all sharply increased above 400oC. The peak value (4.2X10-4) of ZT appears in original SiC/Si composite at about 500oC, which is increased more than 1000 times than that at room temperature.