The LASSO synthetic control method relaxes restrictions on the weights and the intercept of the original synthetic control method so as to improve the prediction of counterfactual outcomes. We apply the LASSO synthetic control method to assess the Kyoto Protocol’s effect on CO2 emission reduction of 23 countries and the European Union, which consists of 15 member states at that time, with binding emission targets under the Kyoto Protocol. We cannot find the statistically significant negative treatment effect for most countries in the following years after the Kyoto Protocol entered into force. As for averaging yearly treatment effects for each country, we find that the average treatment effect for most countries is negative, but it is not statistically significant. In summary, we find little evidence for a significant CO2 emission reduction from 2005 to 2012 for most countries with binding emission targets under the Kyoto Protocol.