Abstract: 【Objective】The nutrient controlled/slow release characteristics and mechanisms of super absorbent polymer(SAP) coated urea(SAPCU) was studied. 【Method】 Series products of SAPCUs with single membrane of SAP or with duplex membranes of modified mineral inside and SAP outside were developed by using different types and different water absorbent rates of SAPs as coating material, and big granule urea and modified mineral coated urea(ZECU and PRCU) as the core fertilizers. The structure of the fertilizers were studied by using Scaning Electronic Microscopy(SEM), and the nutrient release characteristics were studied by using water dissolution rate method and soil column leaching method.【Result】The 24 hours N water dissolution rates of SAPCUs were 18.22%~83.87%, and N accumulated leaching rates in four weeks were 40.91%~65.38%, which had 5.21%~40.68% decrease compared with that of urea under same experiment conditions. The capabilities in nutrient controlled/slow release depended on type, water absorbent rate and coating rate of SAP. Urea coated with copolymer of polyacrylamide and polyacrylate had greater capabilities than those with polyacrylate. Both of water and soil dissolution rates of SAPCUs decreased obviously along with the increase of SAP coating rate. However, SAPCUs with duplex membranes had much better effects on nutrient controlled/slow release than those with single membrane, and the effects were also related to the material of inner membrane. Stereoscan photographs showed that both the inner and outer coating membranes of SAPCUs were formed by random closely stacking different size and shape micro particles of coating materials. Furthermore, adhesive cemented the particles and filled up the gaps among them, but there were still micro holes, which served as the channels for urea dissolving out, in and between the stacking layers.【Conclusion】According to the membrane structure characters and nutrient release mechanism, the author thought that SAPCUs had the characteristics of store type of physical controlled-release system, gel controlled-release system and chemical controlled-release system, and had good capabilities in nutrient controlled/slow release. Therefore, SAPCUs are new controlled/slow release fertilizers with good prospect of development and application.