The effect of particle angularity on the strength and dilation of granular materials is investigated through a series of laboratory tests on two materials, Ottawa standard sand (Sand O) and crushed limestone (Sand L), that are made up of rounded and angular particles, respectively. Triaxial tests on both materials at different confining pressures and initial void ratios show that particle angularity has a substantial effect on both the peak friction angle ϕp and the mobilized friction angle at the onset of dilation, ϕf. It is found that ϕf is smaller than the critical friction angle ϕcv for Ottawa sand; nevertheless ϕf is larger than ϕcv for Sand L owing to interparticle locking induced by particle angularity. The experimental results clearly show the contributions to shear resistance from both dilation and interlocking, with interlocking still largely existing at the peak stress ratio but not at the critical state. Suggestions are made to modify the stress–dilatancy formulations for sand to take into account the effect of interparticle locking associated with particle angularity.Key words: granular material, dilatancy, interlocking, and particle shape.