Abstract: Atmospheric fine particulate matter (PM_2.5) is a key pollutant affecting air quality in the Yangtze River Delta (YRD)．With implementation of various environmental protection measures in recent years, sources of PM_2.5 may have changed．To develop an effective PM_2.5 control strategy, it is vital to identify the main sources of PM_2.5．The CMAQ-ISAM model was used here to analyze quantitatively main PM_2.5 sources in Shanghai, Hangzhou, Nanjing and Hefei in 2018．The major source of PM_2.5 in these 4 cities was found to be long-distance transfer from outside of the YRD region in winter (38.5%-52.6%) but was local emissions in the autumn (43.0%-50.9%)．Local emissions were the main source of PM_2.5 in Shanghai, Hangzhou and Hefei in spring and summer (spring: 37.1%-53.3%; summer: 44.1%-64.7%), but transfer from surrounding areas was the main source in Nanjing in spring (38.5%) and summer (46.3%)．Further source identification in different (clean, pollution) periods within the winter season revealed that, compared with clean period, in the pollution period, contribution ratios from surrounding areas increased, by 10.0%, 1.5%, 8.1% and 4.9% in Shanghai, Hangzhou, Nanjing and Hefei respectively．Therefore it is important to carry out joint preventions and controls both within and without of the YRD region．