Biodiesel is considered as one of the most potential alternative fuels for transport sector. To better assess the overall effects of biodiesel application, an experimental study was conducted concerning the effects of rapeseed oil methyl ester (RME) and its blends with baseline diesel on particle size distribution (PSD) of a turbocharged compression-ignited (CI) engine under both steady and transient operating conditions. The influence of fuel-side combustion technique of using oxygenated additives on the transient particle emission was identified. Tests were carried out under constant engine speed and various load conditions, and controlled transient operation was employed for the transient study. The results show that biodiesel addition causes advanced start of combustion (SOC) under constant injection timing and higher in-cylinder pressure and heat release rate. Biodiesel has significant effects on the particle size distribution (PSD) under both steady-state and transient condition. For steady-state condition, the number concentration of carbonaceous particles (d(p) > 22 nm) decrease while that of non-carbonaceous particles (d(p) < 22 nm) increase with the increase of biodiesel proportion. For transient condition, biodiesel addition, as an oxygenated blending agent, can effective suppress the increase of carbonaceous particles but the nuclei mode particle increase dramatically during the load increasing process. In general, the oxygenated biodiesel is a very potential alternative of fossil fuel in constraining engine smoke opacity and particle emission under both steady-state and transient operating conditions. It is especially effective in reducing larger particles in the carbonaceous soot mode.