The effect of SCR on nanoparticle emissions has been a subject for some recent diesel particle emission related studies. In this study, the effect of after-treatment (DOC and SCR) on particle emissions was studied with a heavy-duty off-road diesel engine (emission level stage 3b with an SCR). A special “transient cold test cycle” (TCTC) was designed to describe the SCR system operation at low exhaust gas temperatures. The particle instrumentation made it possible to measure on-line the particle number concentration, particle size distribution and chemical composition of particles. The largest particle number concentrations were measured after the exhaust manifold. The exhaust after-treatment was observed to reduce the total particle number concentration by 82.5% with the DOC and 95.7% with the DOC+SCR. Also the mean particle composition was affected by the after-treatment; while the organics formed the main fraction of the particulate mass without exhaust after-treatment, after the DOC and especially after the DOC+SCR the organic particulate matter was reduced so that most significant particle fraction was soot. The effect of after-treatment was similar both on exhaust particle number and mass concentration of organics. The SCR system itself was not observed to cause an increase in particle number concentrations by promoting new particle formation. In general, the results indicate that the semi-volatile organic fraction, formed in exhaust cooling and dilution process, can play a significant role in the PM emissions of a modern off-road diesel engine in low exhaust temperature situations. On the other hand, the semi-volatile components, which participate in gas-to-particle conversion during cooling and dilution, were effectively removed by exhaust after-treatment.