Using only chemical without organic fertilizers for intensive paddy rice production in the Mekong Delta, Vietnam has likely caused decrease of soil fertility and rice yield. To recycle nutrients and improve soil fertility, rice straw removed from rice field was composted 1.5 months with Trichoderma sp. inoculated for straw decomposing enhancement, and then was incorporated into rice field at 6 Mg.ha^-1 (moisture 75-78%) for each crop. Field trial at Omon, Vietnam with double cropped rice using rice straw compost has been continuing more than 5 years. It showed that combination of straw compost with 60% of present recommended rate NPK in the region gave higher yield than solo chemical fertilizers at the recommended rate though the difference was not significant. Nitrogen recovery from urea measured by ¹⁵N indicated the recovery rate was not significantly different between solo application of chemical fertilizer and reduced rates of chemical fertilizers combined with straw compost. While soil phosphorus and exchangeable potassium remained unchangeable, soil nitrogen increased under straw compost application compared to full rate of NPK. Higher rice yield of combination of straw compost and chemical fertilizers likely related to better way of N supply. Application of full rate of NPK within 30 days after sowing (DAS) evidently caused N excess in rice. Meanwhile, under combination of compost and chemical fertilizers, having large soil nitrogen pool and more microbial activity, rice could be supplied gradually and stably with more available nitrogen from mineralization of soil organic matter and applied compost, particularly after 30 DAS. Results of anaerobic incubation showed that available nitrogen mineralized from soil in 30 days was higher than applied nitrogen of full rate of urea (100 kg N.ha^-1). Obtained results showed that application of rice straw compost to paddy rice field was a way to improve soil fertility and to reduce chemical fertilizers applied. Key words: rice straw compost, nitrogen recovery, mineralized nitrogen, soil nitrogen pool, anaerobic incubation