Motor vehicle emission factors of carbon monoxide (CO) and non-methane volatile organic compounds (NMVOCs) were calculated inside the Chung-Cheng Tunnel of Kaohsiung in Taiwan. The results were compared with those model predictions from the Mobile Taiwan 2.0 model. Individual concentrations of 21 species of NMVOCs were also determined. Photochemical potential of NMVOCs was evaluated by using the maximum incremental reactivity (MIR). Field data showed that the integrated emission factors of CO and NMVOCs for actual fleet were 6.3 and 1.5 g/veh km, respectively. The error range of these factors may be up to 45%. The predicted values by the Mobile Taiwan 2.0 model closely matched the observed data. Concentrations of isopentane, 2-methylpentane, toluene and m,p-xylene were the dominant species of NMVOCs. The ratio of maximum incremental reactivity to NMVOCs concentration was 3.9, similar to those of the studies in the US Fort McHenry and Tuscarora Tunnel.