Abstract: The PFC is used to study the shear properties of infilled rock discontinuities, with emphasis on the effects of grain shape reflected by the ratio of major- to senior-axis (a/b) and combination. The results show that: (1) Grain movement and failure can be divided into five types, rolling, rolling-sliding, crushing-rolling, comminuting and rolling-crushing, depending on the normal stress and grain shape, and the microcrack evolution is different under each failure mode. The fragmentation degree of grains increases with the increasing normal stress on the whole, while it decreases with the increasing a/b. The abrasion of the discontinuity walls is more serious at higher normal stress and a/b. The tension microcracks are the dominant failure for the grains and discontinuity walls. The size distribution of clastic mixtures formed after grain crushing can be described by a power law exponent or fractal dimension D. The smaller the value of D, the lower the fragmentation degree. (2) When the shapes of the double infilled grains are identical, the surface micro-roughness affects the shear properties to a large extent. When they are different, the grains with higher a/b afford more compression and shear loads, resulting in a higher fragmentation degree. The friction between two grains also affects its surface abrasions and shear movement and failure processes. (3) The average frictional strength of infilled rock discontinuities is related to the size distribution of the clastic mixture. As a/b increases or normal stress decreases, the content of large angular fragments increase, which further leads to the increase in the overall frictional strength. An empirical formula for predicting average friction strengths of rock discontinuities infilled with grains is proposed and is preliminarily validated through the data in the literatures.