In recent years, trans-esterified vegetable oils have been widely applied to diesel engine in order to suppress greenhouse gas emissions. However, “neat” vegetable oils are expected to be directly used to resolve some difficulties faced in their use, such high viscosity and slightly high fuel consumption. In this study neat linseed oil has been investigated as a neat vegetable oil. It was found to show higher fuel consumption than diesel fuel, however at the same time it showed lower indicated fuel consumption than diesel fuel. These results suggest some increase in engine friction loss in a neat biofuel diesel engine. Studies have been extensively investigated the difference in friction loss and a newly developed “improved deceleration method” has been applied. Doing so confirmed the higher fuel injection driving torque caused by the higher viscosity of neat linseed oil relative to diesel fuel and the effect of the enlarged inner diameter of the fuel injection pipe on reducing the engine friction of high-viscosity fuel. Finally, a neat linseed oil engine with an optimized fuel injection pipe showed lower fuel consumption than a diesel fuel engine. The enlargement of the fuel injection pipe and fuel heating which may be more relevant for high-viscosity fuel, were compared from the view- point of fuel injection driving torque to organize the results of fuel consumption improvement in a neat biofuel diesel engine.