The optimum condition for alkaline leaching of roasted zinc concentrate has been studied, The increase in baffle number increased the leaching rate and there existed an optimum pulp density for the highest Leaching rate. It was essential to secure complete suspension of particles with a minimum power consumption in It was required to decrease the vortex depth over complete suspension to The agitation speed for complete suspension (N-s) decreased as the agitator diameter (D) increased, but the dependence of N-s on D increased as the clearance ratio increased.