Field evaluations of slow-release N fertilizers are needed to improve N use efficiency while reducing NH3 volatilization. Ammonia volatilization losses, floodwater NH4+, and aboveground rice (Oryza sativa L.) tissue N accumulation were compared between non-coated urea (NCU) and polyolefin-coated urea (POCU) applications in a double-rice production system of subtropical China. A factorial treatment design included two urea sources such as POCU and NCU applied at 75 and 150 kg N ha-1, and a no-N control. For early and late rice seasons, floodwater NH4+ and pH increased significantly within 7 to 10 d after NCU applications and then decreased rapidly. Nitrogen losses through NH3 volatilization accounted for 16 to 30% and 4 to 8% of the total N applied for NCU and POCU, respectively. Applications of POCU significantly increased N accumulation in aboveground rice biomass due to a slower N release and better synchrony between in-season rice N demand and N supply from this fertilizer. Applications of POCU at a rate of 75 kg N ha-1 provided the optimal amounts of N for early and late rice crops and reduced NH3 volatilization losses. The use of slow release urea fertilizers should increase N use efficiency and maintain the high grain yield in the double rice production systems of subtropical China. © 2013 by the American Society of Agronomy.