The effect of 15 ppm L5Ni/Bis-[N-pyrrolidine-N'-2-chloro-benzoylthioureato] nickel (II) additive in the diesel fuel to the engine performance (brake thermal efficiency and specific fuel consumption), in-cylinder combustion (in-cylinder pressure, temperature and heat dissipation) and exhaust emissions (HC, NOx and CO2) has been investigated experimentally. The experiments have been conducted in a four-cylinder, four-stroke, water-cooled and 1,5 liter common-rail diesel engine. Experimental studies have been conveyed with 1750 rpm constant engine speed, with 40, 60 and 80 Nm fixed engine loads and with variable air-fuel mixtures that change between lambda=3-5 according to engine load. As a result of experimental studies, it has been determined that L5Ni additive to diesel fuel caused an improvement that varied between 0.3-0.8% to thermal efficiency and specific fuel consumption alongside with increasing engine load. Besides, the engine load varied with L5Ni addition to the diesel fuel, however, HC emission decreased about 7, 14-29,41%; NOx emission decreased about 3.78-7.02%; and CO2 emission decreased about 2.29-6.46%. To conclude, 15 ppm L5Ni additive to the diesel fuel provided some improvements in thermal efficiency of the engine and specific fuel consumption, while causing significant decrease in harmful exhaust emissions.