Aim: This work examines the brake thermal efficiency (BTE) of diesel engine runs by biodiesel and diesel blends comprising 20% of cashew nut shell oil (CNSO20D80) diesel blend by volume. Materials and Methods: The test fuels were studied in a four-cylinder and four-stroke unmodified diesel engine at various operating conditions. CNSO20D80 was prepared by combining 80% neat diesel and 20% cashew nut shell oil with some SPAN 80 in it which acts as a surfactant. For each fuel 20 trails were considered based on the G-POWER calculator by having the alpha value 0.05 under 95% confidence level. Results: The BTE at 100% load for CNSO20D80 is 26.56% while the BTE of the diesel is 29.5%. The obtained significance value for fuel trails is 0.029 which is a way less than 0.05, from this there is a significant difference between the fuel groups. Conclusion: The thermal efficiency of the biodiesel is less than the diesel. There is a reduction of BTE by 9.96% for CNSO20D80 when compared to diesel. From this study it is concluded that diesel is more effective when compared to CNSO20D80.