Since diesel engines have higher thermal efficiency, larger power and better fuel economy than gasoline engines, diesel engines are widely used in vehicle, construction machinery and agricultural machinery. However, they emit more hazardous pollutants than gasoline engines, especially particulate emission, which has negative impact on human health and environment quality. In order to meet future increasingly stringent regulations for particulate emission, exhaust gas after-treatment technologies of diesel engines are essential. Particulate emissions from a heavy-duty diesel engine which meets the China national V emission regulation were studied, and the engine was equipped with/without diesel oxidation catalyst (DOC), catalyzed diesel particulate filter (CDPF) and selective catalytic reduction (SCR). The fuel used in this article is ultra low sulfur diesel fuel whose sulfur content is less than 10 ppm. A TSI 3090 Engine Exhaust Particulate Sizer is used to study number and size distribution of particulate emission. Results show that with the increase of engine load, the total number of particulate emission increases, and especially under higher engine load, the increase of total particulate number is drastic in consequence of the deterioration of combustion in the cylinder. Compared to the engine without exhaust gas after-treatment devices, DOC+CDPF+SCR can evidently reduce particulate number by 1~2 orders of magnitude, and the decreasing range can up to 2~3 orders of magnitude when the particulate diameter is between 10~60nm. At the maximum torque speed of 1400r/min and rated speed of 2200r/min, the purification efficiency of total particulate number from the engine can be over about 95%.