In the present study, heat transfer characteristics of oil flow over transverse array of offset strip fins are predicted by the numerical methods. Oil flow in the plate-fin passage is idealized by three dimensions. The governing equations are discretized by finite volume method. The flow patterns and heat transfer characteristics are predicted in details. The convective heat transfer coefficients and friction factor are reduced from numerical results. The effect of variable viscosity on the convective heat transfer coefficient and friction factors is predicted and compared with the results of constant viscosity. The characteristic length which represented the flow situation is selected by comparing friction factors in the case of constant viscosity. Therefore, the equivalence length is selected to the characteristic length. Friction factor is 50% less than that of constant viscosity. And also, convective heat transfer coefficient is 50% more than that of constant viscosity. Correlation for convective heat transfer coefficient is derived.