Abstract: In the summer of 2013，a constant real-time survey of O3 and its precursors ( NOx and CO) near the ground was conducted at ten air quality monitoring sites within Xi'an city． The results showed that the hour average concentration of O3 in the summer was between 39. 03 μg /m3 and 93. 06 μg /m3 ，and the concentration increased from northeast to southwest direction． In addition， the hour concentration distribution of O3 exhibited a distinct mono-peak pattern and the O3 concentration reached the peak at 15: 00 in the afternoon; while the distribution of NOx and CO concentration showed a doublepeak phenomenon， and the CO concentration changed gently compared with NOx concentration． The daily average concentration of O3 was positively associated with the temperature and solar radiation，and negatively associated with relative humidity． The temperature and solar radiation had remankable impacts on the average daily O3 concentration．