Abstract: A gasoline direct injection（GDI） light-duty vehicle was tested to investigate the influence of dilution conditions and measurement systems on number concentration and size distribution of emitted particulate matters. Two dilution and sampling systems and three measurement systems were examined. It was found that most of emitted particulate matters were in accumulation mode, few were in nuclei mode. Geometric mean diameters of these two modes are 22 nm and 83 nm, respectively. Flow rate of the Constant Volume Sampling（CVS） was found to have little effects on particle size distributions. The difference in particle number concentration caused by tested dilution conditions is within ±11%. Number concentrations measured by the PMP system have good repeatability. ELPI+ reports much higher particle number concentrations than those obtained by the PMP system. The differences are 23%~100%. Large discrepancy often occurs at rapidly changing driving conditions. Number concentrations measured by the CVS dilution system are much lower than those by the FPS dilution system.