The first case of COVID-19 infection in Serbia was confirmed in early March 2020. State of Emergency was declared soon afterward, imposing shelter in place measures and curfew in the evening hours. A number of countries around the world have experienced air quality improvements during the pandemic, caused by the mandatory quarantine. This study aims to analyze the effect of the quarantine on the air quality, by comparing PM_2.5, PM₁₀, and NO₂ concentration levels during 32 days quarantine period in the six cities in Serbia, from March 16 to April 16, 2020, with values obtained in the same period in 2019. NO₂ concentration levels showed significant reductions in all cities (36-71%). However, PM_2.5 and PM₁₀ concentration levels did not show significant reductions in all cities. Concentration levels of PM_2.5 and PM₁₀ were lower during the quarantine period only in two cities, by 2 and 19%, and 10 and 30% respectively. Analyzed values can assist decision-makers in defining the levels of effort and scope of actions that need to be undertaken in order to prevent the occurrence of high levels of concentration of PM_2.5 and PM₁₀ and to ensure clean and healthy air in Serbia.