Abstract: Both experiments and theory analysis proved that the Hardgrove grindability index(HGI) of mixed coal was nonlinear with its mixture ratio,which was explained by the theory of micro-grinding-ball effect.The HGI of 5 mixed coal samples was measured in lab.Because the HGI data of different mixed coal samples change in different ranges,they could not be directly compared with each other.Though normalizing by the relative grindability ratio(RGR),the comparison between these data was completed in the same range.The results show that different kinds of mixed coal samples with the same mixture ratio had the same RGR.The formula of RGR was obtained by partitioned least-square polynomial fitting.In conjunction with its definition,a method of calculating the HGI of mixed coal based on the nonlinear hypothesis was proposed.Practical applications indicate that the method is more accurate than that based on the linear hypothesis.