The effect of particle size polydispersity (sigma(D)) on coal dust explosibility was studied using a 20-L spherical explosion test apparatus. Four kinds of blends with a fixed median diameter (D-50) of 95 mu m but varying sigma(D) of 1.18, 1.70, 1.96 and 2.40, were prepared by mixing original coal samples with relative low sigma(D). Experimental results showed that the values of maximum pressure rise (P-ex) and maximum rate of pressure rise ((dP/dt)(ex).V-1/3) of coal blends with the same D-50 of 95 mu m varied largely with different sigma(D), and bigger sigma(D) produced larger P-ex and (dP/dt)(ex).V-1/3 values. The Sauter mean diameter (D-3,D-2) presented the best correlation between particle size and the explosion parameters for the blended coal samples. Risk assessment evaluation of coal dust should be reported in terms of D-3,D-2 and sigma(D) to avoid serious underestimation.