The diesel engine is an internal combustion engine that converts the chemical energy in the fuel into kinetic energy. Waste vegetable oils can be used as an alternative fuel in conventional diesel engines. When waste vegetable oils are used directly as a fuel in a diesel engine; the injection pressure of diesel engines can be altered to help ensure that the viscosity of the vegetable oils is low enough to allow proper atomization of the fuel. Waste vegetable oils can also be blended with diesel fuel for use under a wide range of conditions. The injection pressure is one of the important variables affecting the performance of the engine. In this paper, the effect of changing the injection pressure from 170 to 200 bars on the performance of diesel engine was studied by using different types of blends of waste vegetable oils with diesel fuel. From the most important results of increasing in the injection pressure is improving the engine performance by increasing the brake thermal efficiency in a range of percentages from 3.8% to 19% compared to the 170 bar injection pressure at different loads and reducing the brake specific fuel consumption in a range of percentages from 3.5% to 13% compared to the 170 bar injection pressure at different loads thus increasing the air-fuel ratio in a range of percentages from 3.5% to 19% compared to the 170 bar injection pressure at different loads. The use of these oils in engines helps to reduce dependence on conventional fuels, which leads to preserving the climate, in addition to the fact that re-processing and reusing these oils is harmful to human health.