In this paper, a numerical simulation study was conducted to explore the possibility of an alternative approach: direct aqueous urea solution injection on the reduction of NOx emissions of a biodiesel fueled diesel engine. Simulation studies were performed using the 3D CFD simulation software KIVA4 coupled with CHEMKIN II code for pure biodiesel combustion under realistic engine operating conditions of 2400 rpm and 100% load. The chemical behaviors of the NO formation and urea/NOx interaction processes were modeled by a modified extended Zeldovich mechanism and urea/NO interaction sub-mechanism. To ensure an efficient NOx reduction process, various aqueous urea injection strategies in terms of post injection timing, injection angle, and injection rate and urea mass fraction were carefully examined. The simulation results revealed that among all the four post injection timings (10 ATDC, 15 ATDC, 20 ATDC and 25 ATDC) that were evaluated, 15 ATDC post injection timing consistently demonstrated a lower NO emission level. The orientation of the aqueous urea injection was also shown to play a critical role in determining the NOx removal efficiency, and 50 degrees injection angle was determined to be the optimal injection orientation which gave the most NOx reduction. In addition, both the urea/water ratio and aqueous urea injection rate demonstrated important roles which affected the thermal decomposition of urea into ammonia and the subsequent NOx removal process, and it was suggested that 50% urea mass fraction and 40% injection rate presented the lowest NO emission levels. At last, with the optimized injection strategy, the engine presented significantly reduced NO emission from 131 ppm to 55 ppm compared to the case of pure biodiesel combustion, achieving a reduction efficiency of 58%, suggesting that urea direct injection could be an effective method to reduce the NOx emissions of a biodiesel fueled diesel engine. (C) 2015 Elsevier Ltd. All rights reserved.