The influence of injection timing on exhaust particle size and nanostructure was investigated at two different engine loads on a heavy-duty diesel engine. A Scanning Mobility Particle Sizer (SMPS) and a Transmission electron microscope (TEM) were employed in this experiment. Based on the SMPS results, the concentrations of accumulation mode particles are changed in different ways at the two tested load conditions. Retarding injection timing improves the premixed combustion at the low load condition, which results in a reduction of accumulation mode particle number, while more intense diffusion combustion occurs with retarding injection timing at the high load condition, and thus more carbonaceous material is produced. As measured from TEM images, the primary particle size is reduced with advancing injection timing due to the increased in-cylinder oxidation time. With regard to primary, soot particle nanostructure, no appreciable changes can be found with different injection timings at the low load condition, while particles with less reactive carbon layers are created with the retarded injection timing at the high load condition. (C) 2014 Elsevier Ltd. All rights reserved.