Vegetable oils have been seen as promising surrogates to petroleum diesel in compression ignition internal combustion engines, showing similar performance and combustion characteristics of the fossil fuel. Nevertheless, the use of straight (crude) vegetable oil (SVO) is unfavorable due to its high viscosity, which affects the Sauter Mean Diameter of fuel spray and, consequently, fuel-air mixing process, resulting in incomplete combustion. The SVO heating, as well as transesterification and blending with diesel or additives, are some of the techniques to reduce its viscosity and enable its use. Of these the most simple and direct is the heating and was used in this paper to evaluate the performance and emissions of a diesel engine fueled with preheated soybean oil (PSO) by electrical resistances. The experiments were carried out in a single cylinder four-stroke compression ignition engine with mechanical fuel injection. Different engine speed, load and fuel temperatures were investigated. The fuel temperature was controlled by a PID controller. Performance data such as indicate specific fuel consumption (ISFC), brake torque and indicate thermal efficiency were obtained. Emissions of unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) were measured too. Combustion characteristics were evaluated using heat release rate (HRR) analysis. Results compared to diesel fuel show that ISFC and indicated efficiency are higher for PSO. HRR diagram are similar for diesel fuel and PSO. HC and CO are lower for PSO and NOx is similar for both fuels.