Rapid growth of industrialization, transportation, and urbanization has caused increasing emissions of ozone (O 3 ) precursors recently, enhancing the O 3 formation in eastern China. We show here that eastern China has experienced widespread and persistent O 3 pollution from April to September 2015 based on the O 3 observations in 223 cities. The observed maximum 1 h O 3 concentrations exceed 200 µg m −3 in almost all the cities, 400 µg m −3 in more than 25 % of the cities, and even 800 µg m −3 in six cities in eastern China. The average daily maximum 1 h O 3 concentrations are more than 160 µg m −3 in 45 % of the cities, and the 1 h O 3 concentrations of 200 µg m −3 have been exceeded on over 10 % of days from April to September in 129 cities. Analyses of pollutant observations from 2013 to 2015 have shown that the concentrations of CO, SO 2 , NO 2 , and PM 2.5 from April to September in eastern China have considerably decreased, but the O 3 concentrations have increased by 9.9 %. A widespread and severe O 3 pollution episode from 22 to 28 May 2015 in eastern China has been simulated using the Weather Research and Forecasting model coupled to chemistry (WRF-CHEM) to evaluate the O 3 contribution of biogenic and various anthropogenic sources. The model generally performs reasonably well in simulating the temporal variations and spatial distributions of near-surface O 3 concentrations. Using the factor separation approach, sensitivity studies have indicated that the industry source plays the most important role in the O 3 formation and constitutes the culprit of the severe O 3 pollution in eastern China. The transportation source contributes considerably to the O 3 formation, and the O 3 contribution of the residential source is not significant generally. The biogenic source provides a background O 3 source, and also plays an important role in the south of eastern China. Further model studies are needed to comprehensively investigate O 3 formation for supporting the design and implementation of O 3 control strategies, considering rapid changes of emission inventories and photolysis caused by the Atmospheric Pollution Prevention and Control Action Plan released by the Chinese State Council in 2013.