With the increasing researches on geotechnical properties of the diesel contaminated soil (DCS)，the water content measured is indispensable part during the early period． In this study，the relative error of water content measurement using the traditional method is as high as 20． 78%，which is no longer suitable for contam- inated soil． Through a series of tests to measure the loss coefficient of diesel in the drying time，the authors fi- nally proposed a modified calculation formula for test samples． The results show that the maximum relative error calculated by using the modified formula is 0． 96%，far lower than that of traditional formula，which can pro- vide accurate data for further study of diesel contaminated soil．