Fuel blending is one of the methods of lowering plant oil’s viscosity, which is the major parameter that leads to serious problems on diesel engine performance in terms of operation and durability. A study was carried out on fuel properties of Jatropha (JO) and Croton (CROT) seed oils blended with a distillery by-product, fusel oil (FO) (modifier) of between 20% and 50% (v/v) at room temperature (30 ± 3˚C). The effect of FO composition in the resulting blend on the kinematic viscosity and fuel combustion properties of the blends was analysed. The results were compared to the properties of crude plant oils (POs) and diesel fuel (grade-4D) specifications. The blends were found to be compatible over a wide range of compositions and exhibited much lower viscosity and better combustion properties than crude plant oil (PO) samples. The optimum proportions for a blend’s viscosity to meet the specifications for diesel fuel (grade 4-D) were found to depend on the modifier characteristics; and that 20% (v/v) FO was enough to produce the desired liquid biofuel blend. The characteristics of blends presented in this work matches well with the characteristics of fuels for low- and medium-speed diesel engines. This implies that the PO/FO blends can be used as alternative fuels in low- and medium speed diesel engines.