Abstract: The lag period of nutrient release from polymer-coated NPK compound fertilizer was defined as the period in which the cumulative release of nutrients was almost zero. The lag period came from two processes: water was diffused (into) fertilizer granule, and the void was full of water; the nutrients were dissolved in the water in the granule void. (According) the two processes the main parameters that affect the lag period included membrane thickness, nutrients saturated concentration and the ion composition. Based on these results the lag period was modeled and expressed as following formula: tlag=kr/3(l/PhP+1/D). Polymer-coated NPK compound fertilizers (Multicote), which were provided by Haifa Chemical, were used in this release experiment in water, and the lag period of the release of potassium, nitrate, ammonium and phosphorus was evaluated . Because of the difference of nutrients dissolving properties and their interactions the lag period for varying elements differed significantly, with a longer lag period for phosphorus. The modeled (results) were in line with the experimental results . In addition, this model also showed that the process of nutrients (dissolving) mainly controlled the length of lag period when the thickness of coated membrane was very thin and the coated membrane could control the lag period only when it was thick enough. Therefore, this model could be used to provide theoretical basis for the development and study of polymer-coated fertilizers.