Air pollution caused by diesel engine emissions, especially particulate matters and nitric oxides emissions, is one of the biggest problems of current transportation. In the near future the emission of diesel particulate matters will become one of the most important factors that will affect the trend of engine development. Ambient airborne particles have adverse environmental and health effects and therefore their concentration in the air is regulated. Recent medical studies showed that different particle properties are important (for example: number/concentration, active surface, chemical composition/morphology) and may take role in the responsibility for their human health impact. Thus, diesel engines are one of the most important sources of particles in the atmosphere, especially in urban areas. Studying health effects and diesel engine particulate properties, it has been concluded that they are a complex mixture of solids and liquids. Biological activity of particulate matter may be related to particle sizes and their number. The paper presents the activities of UN-ECE working group PMP on defining the best procedure and methodology for the measurement of passenger cars diesel engines particle mass and number concentrations. The results of inter-laboratory emissions testing are presented for different engine technologies with special attention on repeatability and reproducibility of measured data. .