The estimation of shear strength of rock mass discontinuity is always a focal,but difficult,problem in the field of geotechnical engineering.Considering the disadvantages and limitation of existing estimation methods,a new approach based on the shadow area percentage (SAP) that can be used to quantify surface roughness is proposed in this article.Firstly,by the help of laser scanning technique,the three-dimensional model of the surface of rock discontinuity was established.Secondly,a light source was simulated,and there would be some shadows produced on the model surface.Thirdly,to obtain the value of SAP of each specimen,the shadow detection technique was introduced for use.Fourthly,compared with the result from direct shear testing and based on statistics,an empirical formula was found among SAP,normal stress,and shear strength.Data of Yujian (鱼简) River were used as an example,and the following conclusions have been made.(1) In the case of equal normal stress,the peak shear stress is positively proportional to the SAP.(2) The formula for estimating was derived,and the predictions of peak-shear strength made with this equation well agreed with the experimental results obtained in laboratory tests. The estimation of shear strength of rock mass discontinuity is always a focal, but difficult, problem in the field of geotechnical engineering. Considents of the defects and limitation of existing estimation methods, a new approach based on the shadow area percentage (SAP) that can be used to quantify surface roughness is proposed in this article. Firstly, by the help of laser scanning technique, the three-dimensional model of the surface of rock discontinuity was established. Secondarily, a light source was simulated, and there would be some shadows produced on the model surface. Thirdly, to obtain the value of SAP of each specimen, the shadow detection technique was introduced for use. Fourthly, compared with the result from direct shear testing and based on statistics, an empirical formula was found among SAP, normal Stress and shear strength. Data of Yujian River were used as an example, and the following conclusions have been made. (1) In the case of equal normal stress, the peak shear stres s is positively proportional to the SAP. (2) The formula for estimating was derived, and the predictions of peak-shear strength made with this equation well agreed with the experimental results obtained in laboratory tests.