Background and purpose: In this study, the air quality index and concentration of particles such as PM10 and PM2.5 were investigated in Arak. Materials and Methods: To determine the concentration of PM10 and PM2.5, 60 samples were collected by laser TSI model 8520 in summer and winter 2012. The collection site was around Arak city center. Results: during the sampling period, as a matter of PM10, the cleanest and the most polluted month were December and June with the average of 34.33 µg m-3 and 100.1 µg m-3, respectively. The concentration of PM2.5 was 12.93 and 53.17 µg m-3 for December and June, respectively. Meanwhile, in terms of air quality index (AQI), in 98.3% and 70% of cases, the concentrations of PM10 and PM2.5, respectively were less than normal (AQI100). Conclusion: The concentration of PM10 in the study period was less than Environmental Protection Agency (EPA) 2006 guideline. Meanwhile, the concentrations of PM2.5 in 30% of air samples were greater than EPA guideline. The average PM2.5/PM10 ratio during the sampling period was 0.41 compared to range 0.15 to 0.25 reported by EPA.