As increased in density, the hardness of B4C was heightened remarkably. The effect of CaB6 addition on the hardness was illustrated in Fig. 4. It can be seen that hardness rose in value with an increasing of CaB6 up to15 wt% and a decrease at higher amount. This increase in value was presumably because of a decrease in porosity down to 1% or lesser. As we all know that the pores in a body have a negative impact on the hardness. The decrease in hardness of the specimens with more than 15wt% CaB6 was found to be related to the rule of mixtures. Hardness of CaB6 was lower than B4C, so adding CaB6 naturally decreased the hardness of B4C. Besides, the difference in thermal expansion coefficients of B4C and CaB6 (4.5×10-6/ºC for B4C and 6.5×10-6/ºC for CaB6 ) was also an another reason for reducing the hardness. Sigl [9] noted that microcracks produced by thermal mismatch had a pronounced effect in decreasing the hardness.