Rheological properties, morphology, and oil resistance in natural rubber (NR)/nitrile rubber (NBR) blends were investigated as functions of blending conditions and viscosity ratios of the blends. As for the blending condition effects, Mooney viscosity of the blends depended more strongly on blending time than rotor speed. Size of the NR dispersed phase was approximately independent of rotor speed but decreased with increasing blending time up to 25 min. As blending time further increased, NR dispersed phase size increased. The results of relative tensile strength, which is an indicator for oil resistance, in this study were in agreement with those of the blend morphology, indicating that the oil resistance in 20/80 NR/NBR blend depended strongly on the phase morphology of the blend. The smaller the size of NR dispersed phase was, the higher was the resistance to oil of the blend. However, a decrease in the size of the dispersed phase by the modification of the viscosity ratio via the use of low-molecular-weight rubber (i.e., liquid natural rubber and epoxidized liquid natural rubber) did not result in an improvement in the oil resistance.