Online monitoring and control of size distribution in any grinding circuit is a vital task in mineral processing plants. Flotation process efficiency depends on the size of feed materials and also fraction of the fine particles e.g. -38 microns. This paper presents a new approach to monitor the size distribution of Lakan Pb/Zn flotation circuit in Iran. A software monitoring size distribution has been developed, based on two main parameters named "n" which is reduction ratio in the mill and "b" which comes from Guadin equation, i.e. W=aDb. "n" depends on mill characters and feed function while "b" depends on ore, instruments, and other properties. Liberation and size fraction studies showed that the d80 of ground material should be -74 microns and weight of passing 38 microns materials should be 35-40%, which is the optimum condition for the flotation recovery. Finally, results obtained from ball mill circuit for "n" and "b" indicated that these parameters were equal to "nBall Mill=80-87" and "bBall Mill=0.84-1.1" at optimum condition. Under this condition, flotation recovery increases up to 75%, which is practically confirmed in Lakan flotation plant.