In previous studies, compound metformin /glipizide elementary osmotic pump tablet was developed. Aim   To discover the mechanism of drug re lease from i.t Methods　Three rate-limiting factors influencing drug re lease from dosage form (the semi-permeable membrane, tablet co re and delivery orifice) were investigated. Results　The influx of water that passed the osmotic pump tablet was almost equal to the metformin release rate, while it was greatly less than the drug dissolution rate from tablet core. The size of orifice from 0.4mm to 0.8mm had no influence on drug release. The osmotic pressure of tablet co re was mainly caused by metformin. Conclusion　From the developed model of osmotic pump system s, it can be seen that only the water influx through the membrane is a rate-limiting step, not tablet cored is solution rate and solution influx, and only when the core dissolution rate is equal to the solution influx, the zero order release is seen in the osmotic pump systems.