In the present study, heat transfer characteristics of oil flow over offset strip fins are predicted by the numerical methods. Oil flow in the plate-fin passage is idealized by 2 and 3 dimensions. The flow patterns and heat transfer characteristics are predicted in details. Numerical results show that the average convective heat transfer coefficients are almost independent on the raws of fins and affected by fin pitches. At the rear face of the fin, there exists minimum point of heat transfer coefficients where stream is separated from the fin surfaces. The convective heat transfer coefficients are effected by separation bubbles which appear at the wake region of offset strip fins. Correlations for friction factor and convective heat transfer coefficient are derived. The effect of natural convection is described by three dimensional analysis. And also the results by 2 dimension and 3 dimensional analysis are compared.