In the study, the effect of hydrogen addition with air taken from the intake manifold at different rates on performance and emissions in a single-cylinder, four-stroke diesel engine has been numerically investigated. The first and second law analyses of thermodynamics are performed for each hydrogen addition condition. Numerical analysis has been performed with ANSYS-Forte, energy and exergy calculations have been carried out according to the analysis results. The results of the analysis show that with the addition of hydrogen, CO emissions decrease while the engine performance and NOx emissions increase.