PURPOSES : This study reveals the social cost of illegal parking. Roadside illegal parking causes additional traffic congestion, air pollution, and traffic accidents. Several cities try to reduce illegal parking for the same reasons and build public parking spaces. However, few studies have been conducted on the effects and social cost of illegal parking problems. METHODS : In this study, a real-world study site was selected for the calibration of the VISSIM micro-simulator parameters, and several scenarios were created for developing illegal parking effect models. According to the simulation results, this study demonstrated five regression models that illustrated the average travel speed drop effects and the differences in emissions. RESULTS : Depending on the scenarios, the travel speed was dropped by 20 km/h–57 km/h and the levels of carbon dioxide and nitrogen oxide were increased by 111 g–279 g and 22.5 g–56.3 g, respectively. CONCLUSIONS : In this study, a methodology for estimating the social cost of illegal parking was established. Therefore, it is expected to contribute to the evaluation of policies related to illegal parking or the feasibility of facility investment.