The effect of biodiesel on the Particulate emissions is gaining significant attention particularly with the drive for the use of alternative fuels. The particulate matter (PM), especially having a diameter less than 50 nm called the Nanoparticles or Nucleation mode particles (NMPs), has been raising concerns about its effect on human health. To better understand the effect of biodiesel and its blends on particulate emissions, steady state tests were conducted on a small-bore single-cylinder high-speed direct-injection research diesel engine. The engine was fueled with Ultra-Low Sulfur Diesel (ULSD or B-00), a blend of 20% soy-derived biodiesel and 80% ULSD on volumetric basis (B-20), B-40, B-60, B-80 and 100% soy-derived biodiesel (B-100), equipped with a common rail injection system, EGR and swirl control systems at a load of 5 bar IMEP and constant engine speed of 1500 rpm. The exhaust sampling was done using a Dekati Fine Particle Sampler (FPS) and the exhaust was diluted to a ratio of 35(±2): 1 and the dilution temperature was maintained around 200°C(±10). The measurement of the particle size distribution was done using Scanning Mobility Particle Sizer (SMPS). Injection pressures of 600 bar, 800 bar, 1000 bar and 1200 bar were used for each of these fuels. The data indicated that increase in the percentage of biodiesel in the blend with ULSD fuel increases the particulate number density and reduces the particulate mass.