Abstract: Measurements of particle number emissions and particle size distributions from a diesel engine under different rotation speeds and loads, and with and without a diesel oxidation catalyst (DOC), were performed using an electrical low-pressure impactor (ELPI). The results showed that the number concentration of particles increased with the speed of the engine regardless of whether or not the DOC was used. With increasing engine load, the peak number concentration occurred at a larger diameter. After the DOC, the particles still mostly showed a unimodal normal distribution, and the DOC had a high conversion efficiency on nucleation mode particles. Under low engine speed conditions, particles at all diameters on average showed lower number concentrations. Under medium and high engine speed conditions, the DOC showed no clear impact on number concentrations of particles greater than 120 nm in diameter.