The stable weight of armor blocks of sloping breakwaters is calculated by Hudson formula, yet it is indefinite for its safety level. The concept of safety factor for armor blocks and its value of KN=2.2 is given in this paper. Model experiments certify that the value of safety factor is appropriate. The safety factor can be used in determining the stable weight of armor blocks, the coefficient of stability for new-type armor blocks, and in checking the rationality of stability coefficient for all kinds of armor blocks.