To design a planar nozzle/diffuser, its divergence angle and slenderness are experimentally investigated in this work according to the flow rates of the assembled micropumps. Thirteen micropumps with different design of nozzle/diffuser were chosen from the no stall region, transitory region, and bistable steady stall region in a complete stability map of diffuser flow patterns. These micropumps actuated by piezoelectric buzzer were fabricated by UV photolithography. The measured volume flow rates indicate that those micropumps having better performance are located in the transitory region where the flow is strongly unsteady. Among them, the best two nozzles/diffusers occur at the line of the maximum pressure recovery.