Use of straight vegetable oil (SVO) directly in the engine creates several operational and durability problems such as severe engine deposits, injector coking, piston ring sticking, gum formation and lubricating oil thickening. These problems relate to the high viscosity, poor volatility and cold flow characteristics of the vegetable oils due to large molecular weight and bulky molecular structure. Researchers have suggested different techniques for reducing the viscosity of the vegetable oils, which are dilution/blending, eating/pyrolysis, micro-emulsification and transesterification. This research paper investigates the viscosity of preheated karanj oil at various temperatures and in running a Kirloskar, twin-cylinder-type, DI diesel engine with preheated karanj oil. The karanj oil was preheated with the help of the hot exhaust gases so that its viscosity reduced and then it was injected into the engine. Investigation on engine were carried out with the aim to compare the performance characteristics such as brake specific fuel consumption(BSFC), brake thermal efficiency, brake power, exhaust gas temperature, emission such as smoke opacity with diesel and preheated karanj oil. Viscosity was measured at 27°C-80°C for selection of fuel suitable for engine. At 80°C, viscosity was comparable with diesel and hence karanj oil was heated up to 80°C and injected into the engine. At lower loads, brake thermal efficiency increased up to 8.79% as compared to diesel, but there was 9% increase in BSFC compared to diesel. It was found that smoke density also increased compared to diesel. But exhaust gas temperature lowered as compared to diesel.