A Monte-Carlo simulation method of rock quality designation (RQD) is presented in this paper based on the negative exponential distribution and lognormal distribution for modeling the discontinuity spacings. First of all, 500000 random structure planes were generated based on the negative exponential or lognormal distribution to form a long scan line according to the generation sequence. Subsequently, a series of scan lines, which are about 2 m long in accordance with the practical drilling engineering, were generated based on the uniformly distributed random sampling on the above long scan line and homologous rock quality designations are computed. Meanwhile, the mean value of RQD and cumulative distribution curves of these RQD values were obtained within the same simulation program and the results are plotted. For most rock masses in the nature the mean value of RQD, rock quality percentage (RQP) and rock quality risk (RQR) can be read off from the rock quality description charts plotted in this paper.