Abstract: Based on the emission inventory of the VOCs in Lanzhou City, petrochemical plants, coating plants and ethylene plants were selected to analyze the fugitive emission characteristics of VOCs using off-line monitoring instruments. The atmospheric chemical reactivity of VOCs and their OFP (ozone formation potential) were evaluated by using MIR (maximum increment reactivity) and L^OH methods (·OH radical reactivity). Also, we studied the diagnostic ratios of the different enterprises. The results showed that the φ(VOCs) varied from 20.8×10^-9 to 6, 520.3×10^-9 in those VOC sources, among which the coating plants were the highest, the lowest was in ethylene plants. As for the compositions of VOCs, aromatics were the most abundant species in coating factories, while alkanes were in petrochemical plants and ethylene plants. Moreover, the processes of petrochemical plants were also slightly different. The photochemical reactivity of VOCs in the coating plants was higher than that in the other industries of this study, with which L^OH and OFP were 2, 676.9 s^-1 and 72, 519.0×10^-9, respectively. It was about 18 to 1, 000 times that of other industries. Moreover, the aromatic hydrocarbons such as m, p-xylene, ethylbenzene and o-xylene accounted for the highest ratio. The petrochemical industry was followed with the L^OH and OFP being 273.2 s^-1 and 4, 039.1×10^-9, and the major contribution came from diesel. N-pentane, isopentane, ethylene and propylene were dominant species for ozone formation in the petrochemical plants. The contribution of ethylene plants was the lowest in the study, with the L^OH and OFP values of 4.6 s^-1 and 69.7×10^-9. Its photochemical reactivity mainly came from olefin species such as ethylene, propylene and n-butene. BTEX (the combination of benzene, toluene, ethylbenzene and three xylenes) ratios showed that some but not all VOC sources could be distinguished by those diagnostic ratios. The research indicated that the control of the industrial emission sources, especially the coatings and petrochemical plants, would be favorable for controlling ozone formation.