The volume of timber is the main basis for nature determination and punishment of the forest cases such as illegal logging and its result must be sole.The data of the down timber to Pinus Massoniana Lamb,Cunninghamia lanceloate(Lamb.)Hook.and Populus Canadensis Moench.are used to calculate their volumes in different formula respectively,the results obtained are as follows:the volumes of sectional measurement formula with mid-section(n>5)are nearly equal to the exact ones and the appropriate error is smaller.So the volume is suitable to be sole as one of forest cases in the state of down timber.