Abstract The effect of particle roughness on turbulence structure in a water boundary layer in an open channel flow is experimentally investigated using a laser-Doppler velocimeter (LDV). As a wall boundary, a fixed- or a free-particle wall is used against various particle-occupation densities ε. The results show that for the same ε the friction velocity on the fixed-particle wall is larger than that on the free-particle wall, and that both friction velocities have maxima in the region of 0.1⩽ ε⩽0.2. The Reynolds stress and turbulence intensities outside the roughness sublayer, normalized by the friction velocity, are independent of the particle roughness. The u– v quadrant decomposition technique elucidates that ejection events are superior to sweep events in the vicinity of the wall over smooth and free-particle walls, whereas over the fixed-particle wall the sweep events are more dominant in the production of the Reynolds stress than the ejection events.