The characteristics of particulate mater size distribution in the exhaust gas of an automotive diesel engine have been studied for a biodiesel blend of 30% rapeseed methyl ester (RME) and 70% ultra low sulphur diesel (ULSD) by volume (B30). The engine, a twin-turbo charged V6 equipped with a common rail fuel injection system, was operated on 16 steady-state points extracted from a corresponding New European Driving Cycle test with no engine system modification and a fast differential mobility spectrometer was used to determine the particulate number concentration and distribution. It is shown that the number-size distribution is dependent on engine operating conditions including the rate of exhaust gas recirculation (EGR). Compared with ULSD, B30 leads to a 41% smaller average size of the particles with EGR but gives rise to a higher number concentration under certain engine operating conditions, with the differences varying between nucleation and accumulation mode. The calculated particle total mass for B30 combustion aerosol is lower than the value with ULSD for all the engine operating conditions tested. The average B30 aerosol was 28% smaller in size on mass basis, compared to ULSD aerosol. For both fuels, the relationship between the particle total number and total mass has been found to be directly correlated and both the number and the mass of particles increase when the mean diameters of particles increase. (C) 2011 Elsevier Ltd. All rights reserved.