In this study, waste frying oil (WFO) was blended with mineral diesel fuel (MDF) (without converting biodiesel) in the ratios of 2%, 7%, 15% and 25% (v/v) on the condition that the viscosities of all WFO-MDF blends were lower than viscosity upper limit given in European Biodiesel Standard, EN 14214. It was aimed to determine the influences of the direct usage of WFO-MDF blends with low viscosity on the performance, injection, combustion, and emission characteristics of a Common Rail Direct Injection (DI) diesel engine. Engine tests were performed at constant engine speed of 2000 rpm and five different engine loads (50 Nm, 75 Nm, 100 Nm, 125 Nm and 150 Nm). WFO blends' brake specific fuel consumption values were higher than those of MDF. The brake thermal efficiencies of MDF are better than WFO-MDF blends. Slightly higher cylinder gas pressures were attained with the neat MDF but the crank angles where the peak pressures obtained were close to each other. Injection timings did not show significant differences, but fuel quantities injected during pilot and main injection were higher for WFO blends. Excluding 50 Nm engine load, ignition delay and combustion durations were consistent with each other. Compared to MDF as the reference fuel, the use of WFO caused to increase in all exhaust emission types measured, especially THC emissions. The deterioration in performance and emission characteristics increased with increasing engine load and WFO content of the fuel blend.