The exhaust emissions from coal-fired power plants have received much attention because coal-fired power plants are the one of the largest sources of particulate matter (PM) emissions in South Korea. To measure the PM10 and PM2.5, we developed the novel diluter which is comprised of ejector and porous tube in series. The dilution ratio must be defined to calculate particle concentrations of the sampled air as well as to probe match for the isokinetic sampling. For this reason, we verified the dilution ratio of the developed diluter by the flow rate, numerical solution, gas concentration and particle concentration. The ejector-supplied flow rates were 10-50 L/min and the porous tube-supplied flow rates were 30, 50 L/min in this study. All methods above showed similar dilution ratios to each other within 10 % error rate. The dilution ratio was confirmed by comparing mass concentrations before and after the dilution process.