On-demand ride-sharing services, such as Uber and Lyft, promote themselves as an innovation that solves old transportation problems and as sustainable transportation systems that reduce traffic congestions and environmental impact. Despite the increasing studies that examine the societal and economic impact of ondemand ride-sharing services, little is known on the environmental impact of these services. Using data collected from 46 Metropolitan Statistical Areas in the United States, we employ difference-in-differences framework to investigate the impact of the entry of an on-demand ride-sharing service, UberX, on gas emission levels. The results suggest an increase in the maximum levels of gas emissions after the introduction of UberX.