Abstract: There existed deviation in determination of the discontinuity trace length in rock mass by using sampling window method, during the field sampling relatively to infinite two-dimensional rock mass. Therefore, the field sampling deviation of trace length has to be corrected in real rock engineering to get the corrected mean trace length. the sampling window method proposed by Prof. Kulatilake in 1984 is briefly reviewed and some modification of it is proposed in this paper. the improved method is more reliable and perfect in calculation. It is convenient to obtain correct discontinuity trace length in infinite two-dimensional rockmass by means of computer program.