After modification, Toufar model was used to calculate the packing degrees of sand mixtures with different particle sizes. For four gradations of sands, the weight ratios of different types of sands with different size ranges, which achieve maximum packing degrees, have been obtained using the modified Toufar model. A strength test of reactive powder concretes (RPCs) with the four gradations of sands was reported. The test results show that the strength of RPC is related to both the maximum grain size and the packing degree of sand mixture. The smaller maximum grain size and larger packing degree of sand mixture can achieve the higher strength of RPC.