In this paper the effects of flow rate, slurry solid content, and feed size distribution on the rod mill efficiency have been investigated. It is difficult to achieve highest grinding efficiency without sufficient information on operating parameters and their effects on the system. Several experiments were performed on a continuous rod milling circuit. Two flowsheets arrangements were considered to produce the particles with size <500 mu m: an open circuit and a closed one. The flow rate selected at range of 11.5-17t/h, slurry solid content at range of 27-61%, and feed size distribution at range of 12-30mm. Results showed that the lowest grinding efficiency was occurred at solid content of 44wt% and deviation from this optimum amount had positive implications on mill efficiency. Results also revealed an inverse relationship between flow rate and reduction ratio. The results of both open and closed circuits showed that the feed size had no effect on the mill product size (80% passing product size or d(80)). However, the reduction ratio was found to be sensitive to the feed size. Mathematical models were also developed to predict d(80) and reduction ratio as a function of flow rate, solid content, and 80% passing feed size (f(80)).