Transformer oil, a source of waste oil that has been suggested as a possible replacement fuel to compression ignition engines, the transformer oil is often used in power equipment to provide insulation and cooling purpose. The intention and main purpose of this manuscript is to use waste transformer oil in a diesel engine, which will prevent environmental issues created by spilling it in open space and sea water. The investigation is performed at waste transformer oil biodiesel at 20% blend (WTOB20) without EGR and its established with variable percentage of exhaust gas recirculation (i.e. WTOB20+5% EGR, WTOB20+10% EGR and WTOB20+15% EGR) for controlling the NOx emission. The investigation's observations are that the overall amount of NOx emissions at full load is reduced by 248ppm with 15% EGR, and the engine that worked Using different levels of BP and an EGR installation, it was discovered that the biofuel oxygen concentration is the biggest factor for reduced NOx emissions. © 2023 Elsevier B.V., All rights reserved.