This study focuses on the effect of particle size of cerium oxide (CeO2) nanoparticle on the emissions characteristics of four-stroke, single cylinder water-cooled diesel engine fueled with neat neem biodiesel (NBD100). Neem oil is transesterified into biodiesel and employed in this work. Cerium oxide as an additive in various particle-size of 10 nm and 20nm is included to NBD100 and termed as NBDCeO(2)10 and NBDCeO(2)20. The experimental result proved that all the chemical and physical properties of fuels are in sequence with ASTM standards. The CO and HC emissions are 4.3% and 4.7% lower for NBD100 than diesel at 3.5 bar BMEP. CeO(2)nanoparticle further reduces CO and HC emissions 4.2% and 3.6% correspondingly for NBDCeO(2)20 than NBD100. The degree of NO(X)emission in NBD100 is 5.6% higher at 3.5 bar BMEP. When compared to NBD100, tailpipe NOx emission was found to be 2.7% and 3.6% lower when fueled with NBDCeO(2)10 and NBDCeO(2)20. In addition, the tailpipe smoke emission was found to be 1.7% lower when fueled with NBD100.