Abstract: Using a classical case, a block on an incline, shear mechanics of discontinuous deformation analysis (DDA) for frictional interface is especially investigated. The results reveal that two contact states are commonly emerged in one edge-to-edge contact, even when the frictional angel is slightly above the theoretical critical friction angle . Owing to that the state of the block will be prevented in the following processes, unreasonable energy will be inevitably produced, which will also make the block hard to get steady. When the Young’s modulus of the block is fixed, carefully selecting a penalty value is required for DDA to stabilize the block when the friction angle is slightly above . A further investigation reveals the main cause is that individually analyzing the two vertex-to-edge contacts in one edge-to-edge contact. Instead individually analyzing the two contacts in one edge-to-edge contact, this study introduces an edge-to-edge treatment to overcome the deficiency of the original DDA. The test results indicate that the new improved DDA can handle well shear problems with high accuracy, and the penalty value and Young’s modulus of the block have little influence on its accuracy.