Based on the differential optical absorption spectroscopy(DOAS) technology, in situ measurement of O₃ and NO₂ was carried out in Chaoyang District of Beijing in summer, 2007. Their diurnal variation characteristics are analyzed. O₃ showed a strong diurnal variation. Due to photochemical reaction, concentration of O₃ is high in day, and the maximum concentration occurs in the afternoon. Its concentration is low at night and the minimum value occurs before sunrise. The contrary trend is observed for NO₂. The concentration of NO₂ exhibits a diurnal variation with low level during daytime for photochemical reaction and high values during the night for traffic emissions. Measurement results are compared with the DOAS system and traditional point monitoring instruments, and good correlation is showed. Based on the sensitivity analysis, it can be found that O₃ production is sensitive to the VOCs.