This paper describes the role of mobilized asperity angles in shear behaviour of weak rock joints based on the results of direct shear tests using roughness profiles in 1mm intervals. Matched joint sets of plaster casts, which simulate Barton's typical joint roughness profiles, are created for the tests. In order to simulate accurate asperities with the same intervals, special moulds are produced by a 3D printing technique. Based on the measured compressive strength of the plaster casts, the direct shear tests are performed under low normal stress conditions. The interpretation of the test data demonstrates a parameter which is the relationship between the mean values of mobilized asperity angles in damaged areas to the asperity component of Barton's shear strength criterion. In low normal stress conditions, Baton's criterion, combined with the proposed parameter, shows high correlation with the test results. This indicates that the shear behaviour of joints is governed by the partly mobilized asperities in low normal stress conditions. As a result, the parameter has a linear relationship with the joint roughness coefficients according to the normal stresses and the compressive strength of the plaster materials. As the parameter is obtained from the asperity angles estimated by 1mm intervals, this can correlate with measured roughness profiles obtained by manual or remote sensing methods. Keywords: JRC, average asperity angles, mobilized asperity parameter, 3D printing