At the present time, the energy crisis, climate and air pollution represent major global problems in addition to the high prices of fossil fuel and vegetable oils. These problems were increased with the current global conflicts which have led to an increase in global warming and rising global temperatures causing significant climate change in many regions of the world. A carbon dioxide emission is one of the main pollutants that cause an increase in global warming. Waste vegetable oils can be used directly as an alternative fuel in diesel engines because they are environmentally friendly. In this paper, the effect of used waste vegetable oils on the emissions of a diesel engine with different blends compared to pure diesel fuel was studied. One of the important results of this research is that with the increase in the proportion of used waste vegetable oil blend in the fuel; it leads to a decrease in carbon dioxide emissions. Carbon dioxide emissions increase by 5 % and 10.13 % for B20-WPKO and B20-WSFO fuels respectively compared to pure diesel fuel at 60 % of load and also carbon dioxide emissions decrease by 12.66 % and 7.6 % for B40-WPKO and B40-WSFO fuels respectively compared to pure diesel fuel at 60% of load. The use of these oils has led to a further decrease in carbon dioxide emissions, and thus a decrease in global warming, furthermore re-processing and reusing these oils is harmful to human health.