Objective To explore the variation of engine exhausts and its secondary pollutant in the daytime. Methods The concentration of engine exhausts(NOx) and its secondary pollutant(O3) were determined during 7:00-17:00, Apr 10th and 12th, 2005. The ratio of air pollutants to vehicle flow (KNOx, KO3) was analyzed to reflect the variation of air pollutants concentration in the daytime. Results The values of KNOx, and KO3 showed the same trend during 7:00-9:00 and 17:00-18:00, the opposite trend during 11:00-15:00. The peak value was observed at 12:00 for KNOx with the lowest level, at 13:00 for KO3 with the highest level. Conclusion Air pollution by secondary pollutant from engine exhausts is associated with sunlight.