The choice of a power plant in different nations depends on the required resources availability, abundance, and reliability. It is evident that renewable energy generation resources provide a more sustainable solution than the fossil fuels. However, as a complete system, renewable energy generation systems also have some overall environmental impacts on humankind and ecosystems. The main objective of this paper is to addresses the environmental effects caused by different types of renewable energy generation systems through process-based life-cycle impact analysis. A comparative study is carried out among different renewable energy generation technologies which include wind, photovoltaic, biomass, and hydro power. Life-cycle impact analysis has been carried out by Institute of Environmental Sciences (CML), Eco-indicator 99, Cumulative Energy Demand (CED) and Ecopoints 97 approaches. The superiority of this work is creating a comprehensive life-cycle inventory following a reliable global database, assessing the impacts for about 10 midpoint impact categories and three endpoint indicators, and taking account of the fossil-fuel-based energy consumption rate of each plant. The key findings reveal that photovoltaic power plants have the highest environmental impacts in the types of ozone layer depletion, fresh water aquatic ecotoxicity and marine aquatic ecotoxicity, while biomass plants impact on the categories of abiotic depletion, global warming, photochemical oxidation, acidification, and eutrophication. Moreover, the wind power plants have the more significant environmental effect on human toxicity and terrestrial ecotoxicity types. Overall, hydropower plants are found to be much more environment-friendly than other renewable electricity generation systems.