Abstract:After the method of mean trace length estimation of rock mass joint was analyzed and summarized,the theory basis of mean trace length estimation by scanline method was extended. The extended scanline method can estimate mean trace length much more exactly in case there is not a great deal of data which are surveyed in field. As for the circular window method,there is a large error between the assumption that the trace midpoint density distribution is uniform in the survey window and the actual density distribution. The window scale must be large enough to guarantee that the distribution of trace midpoint is uniform. The diversity of trace midpoint density decreases;and the estimation of mean trace length is more exact as the radius of circular sample window increases. Abundant joint data were obtained through field survey. The mean trace length of rock mass joint was estimated by the extended scanline method and circular window method. When the diameter of circular window is equal to the censored length with scanline method,the mean trace length estimated by scanline method is close to that estimated one by circular window method. Thus,the mean trace of rock mass joint can be obtained.