Emissions from vessels are a major environmental concern because of their impacts on the deterioration of the environment, especially global warming of the atmosphere. Therefore, the International Maritime Organization (IMO) concern significant care to environmental protection through the reduction of exhaust emission and improvement of energy efficiency through technical and operational measures. Among the suggested measures from IMO, the alternative fuel such as Liquefied Natural Gas (LNG) has the priority to be used instead of fossil fuels. The present paper calculates the effect of using LNG in a dual fuel engine from Environmental and Energy efficiency perspectives. As a case study, a Container Ship has been investigated. The results of the analysis show that percent of CO2, NOx and SOx emissions reduction corresponding to using a dual-fuel engine operating by LNG instead of a diesel engine operating by Heavy Fuel Oil is about 30.1%,81.44%, and 96.94%, respectively. Also, the attained Energy Efficiency Index Value in the case of using the dual-fuel engine is lower than its value by using diesel engine by about 30% and this value will be 77.18%, 86.84%, and 99.27% of the required value of the first, second and third phases, respectively as recommended by IMO.