The article presents experimental test results of a DI single-cylinder, air-cooled diesel engine FL 511 operating with the normal (class 2) diesel fuel (DF), rapeseed oil (RO) and its 10%, 20% and 30% (v/v) blends with aviation-turbine fuel JP-8 (NATO code F-34). The purpose of the research was to analyse the effects of using various rapeseed oil and jet fuel RO90, RO80 and RO70 blends on brake specific fuel consumption, brake thermal efficiency, emissions and smoke of the exhaust. The test results of engine operation with various rapeseed oil and jet fuel blends compared with the respective parameters obtained when operating with neat rapeseed oil and those a straight diesel develops at full (100%) engine load and maximum brake torque speed of 2000 rpm. The research results showed that jet fuel added to rapeseed oil allows to decrease the value of kinematic viscosity making such blends suitable for the diesel engines. Using of rapeseed oil and jet fuel blends proved themselves as an effective measure to maintain fuel-efficient performance of a DI diesel engine. The brake specific fuel consumption decreased by about 6.1% (313.4 g/kW·h) and brake thermal efficiency increase by nearly 1.0% (0.296) compared with the respective values a fully (100%) loaded engine fuelled with pure RO at the same test conditions. The maximum NOx emission was up to 13.7% higher, but the CO emissions and smoke opacity of the exhaust 50.0% and 3.4% lower, respectively, for the engine powered with biofuel blend RO70 compared with those values produced by the combustion of neat rapeseed oil at full (100%) engine load and speed of 2000 rpm.