Introduction: This study aimed to calculate and evaluate air quality index (AQI) in Tehran according to the EPA’ s instruction, and evaluate the obtained values. Materials and methods: The study period included three years of 2013, 2015, and 2017 within the six-year period of 2013-2017. The concentrations of six studied pollutants were acquired from DEPArtment of Environment (DE) and Air Quality Control Company of Tehran (AQCC). EPA’ s method was applied to calculate AQI values during the study period, selecting the highest AQI value among all of the pollutants and monitors. Results: The three-year average of AQI was about 147. During all the three years, the air quality had never been in a “ good” condition (AQI<50). About 92%, 91%, 87% of AQIs indicated the conditions unhealthy for sensitive groups (100