In most of the geotechnical work, mechanical behavior of the rock mass is depends on discontinuities and their properties. Shear tests are the most useful and commonly performed tests to evaluate the shear strength of the discontinuities. The correct shear strength value depends on the proper testing and the accurate interpretation. Discontinuities of the Bakhtiary dam that they are mainly joints and beddings, therefore it was necessary to laboratory and in situ shear test. The results of analysis of 106 samples laboratory shear test and 3 samples in situ shear test to determine the shear strength and development constitutive model. The developed constitutive model in site is Barton’s model. The needed parameters to estimate the shear strength are included JRC, JCS, . To determine the joint roughness coefficient (JRC), initially using the code was developed by the C# programming language and the existing relationships, fractal dimension (D) was determined, and then the JRC value was determined based on existing relationships. Also in this study developed the new relationship for determining the JRC values of D values. In addition, joint roughness coefficient parameter was determined by using this relationship. Then the relationship between the fractal dimension D and the joint roughness coefficient JRC and the asperity angle with Length of the sample was measured, respectively, and a new relationship between them was developed. The results showed that the stationary limit for the parameters D, JRC and i is the estimated to be about 550 mm length. In addition, the residual friction angle was determined using a Mohr-Columbus criterion and joint compressive strength was determined using Schmidt hammer and relationships. Then according to the determinate parameters, constitutive model for major Discontinuities of the Bakhtiary dam that they are joints and beddings were determined. After determining the constitutive model based on laboratory and in situ shear tests, the shear strength of the rock mass in site was determined. Accordingly, due to the lack of shear tests on each scale, tried that initially, laboratory and in situ models were constructed in UDEC software, the results of numerical modeling with using the results of laboratory and in situ tests were calibrated. Then 36 samples of 106 samples laboratory shear test and 3 samples in situ shear test was selected as samples. In addition, the Scale effects in the range of 0.1 to 2 m were measured on shear strength of discontinuity. Finally, the results of 288 samples shear test, the relationship between shear strength and length of discontinuity was evaluated. In addition, an equation for determining the shear strength along ased on shear strength along was developed.