The present study investigates experimentally and theoretically the effects of using of gasoline-diesel fuel blends on single-cylinder diesel engine performance and exhaust emissions. The study consists of two parts: (i) Experimental study: The effects of using of different gasoline-diesel fuel blends have been investigated experimentally and compared in a single-cylinder diesel engine. In this part, the effects of 2-10% gasoline blends have been investigated experimentally in a single-cylinder diesel engine at the speeds of 900-1600 rpm and at the selected compression ratios of 18-23. (ii) Theoretical study: A computer program has been developed for the prediction of diesel engine cycles and engine characteristics for the cases of neat diesel fuel (NDF) and gasoline-diesel fuel blends. For calculation of the diesel engine cycle, a quasi-dimensional phenomenological combustion model developed by Shahed and then improved by Ottikkutti has been used and modified with new assumptions. After the engine cycle model for NDF and gasoline blends was proven to give correct results by comparing it to experimental and theoretical results, 2-10% gasoline blends have been investigated theoretically for the same experimental engine. In the theoretical studies, ignition delay, combustion duration, pressure and temperature of the cylinder charge, and mole fraction of carbon monoxide (CO) and nitric oxide (NO) concentrations have been calculated. From experimental results, it is determined that brake-effective power output decreases at the levels of 1.5-4% at low engine compression ratios, such as 18, 19, and 20. However, brake-effective power increases 1.5-4% at high compression ratios, such as 21, 22, and 23. Brake-effective efficiency increases at the levels of 2-6%, and brake-specific fuel consumption decreases at the levels of 2-6% at low ratios of gasoline/diesel fuel. It can be said that the ratios of 4-6% are the most favorable percentage interval of gasoline at the selected compression ratios for this engine. Because the cost of gasoline is higher than that of diesel fuel and the decrease in the brake-specific fuel consumption is low, gasoline blends are not economical for this engine at low compression ratios. However, approximately 3% price saving is attained at low blend percentages for high compression ratios. From theoretical results, it is determined that ignition delay increases as the gasoline ratio increases because of the decreasing cetane number (CN) of the mixture. Combustion duration remains approximately constant with an increasing gasoline ratio at low engine speeds, but it decreases at high speeds. The mole fraction of CO and NO concentrations increase with an increasing gasoline/diesel ratio because of increasing combustion temperatures. As the gasoline/diesel ratio increases, the NO concentration and mole fraction of CO of this engine increase at the levels of 7-29 and 15-90%, respectively.