The influence of feed concentration on the solid/liquid two-phase flow in a mini-hydrocyclone was studied. A phase doppler particle analyzer was used to measure the two-phase flow pattern in a 25 mm hydrocyclone at three different feed concentrations (300 mg/kg (0.136% (v/v)), 800 mg/kg (0.364% (v/v)) and 1200 mg/kg (0.545% (v/v)). The measurements show that the feed concentration has remarkable influence on the velocities in a hydrocyclone. A higher concentration of solid particles leads to lower axial velocities and can suppress the turbulence of the liquid phase in the inner helical flow; in the outer helical flow, however, the influence was complex. In planes in eddy flow, the downward flow of the liquid phase was increased by a higher concentration of particles; at same time, the dimension of circular flow was also decreasing. In the pyramidal zone, however, the higher feed concentration corresponds to lower axial velocities at the wall region. In the whole experimental zone, the particles lead to the decreasing of tangential velocities. The presence of particles has little influence on the basic flow structure, but changes the size of the eddy flow in the cylindrical section. The correspondence between the higher feed concentration and the shift of the line of zero velocity value closer to the core is also observed which probably means more inlet particles would lead to more liquid leaving the hydrocyclone through the circular flow.