We forced biodiesel to oxidize and heated biodiesel to accelerate its oxidation at about 110° for 10 days. Total particle number and size distribution were investigated in unoxidized and oxidized biodiesel blend fuels. To better understand the effect of biodiesel blends on nano particulate emissions, steady state tests were conducted on a heavy-duty diesel engine. The engine was fueled with Ultra Low Sulfur Diesel (ULSD), a blend of 10% biodiesel derived from waste cooking oil on volumetric basis, equipped with a common rail direct injection system and turbocharger, lives up to the requirements of EURO III. Particle number and size distribution of the diesel engine were obtained by using an Engine Exhaust Particle Sizer (EEPS) and the dilution temperature was maintained around 150° (±5) to avoid condensation of particles. The experimental results show that NOx increase and PM (Particulate Matter) decrease for unoxidized biodiesel blend fuel in comparison to ULSD. Also total particle number increase or decrease with depending on various test conditions. The oxidized biodiesel blend fuel, by and large, decreased NOx, PM and PN when compared with unoxidized biodiesel blend fuel.