The effects of thermal conductivity of fins on the heat-transfer characteristics of offset fins in low-Reynolds-number region are studied. Three-dimensional forced convection flow is solved numerically considering heat conduction in fins, and the results are compared with experimental results. It was found that : (1) heat-transfer coefficient on SUS fins in water is less than that on aluminum fins. This is caused by the fact that the thermal boundary layer which developed on the bottom surface relaxes the temperature gradient in the fluid perpendicular to the fin surface, as well as by the fact that the heat conduction characteristics of SUS fins is poor. (2) Analytical results of pressure drop and heat-transfer coefficients agree very well with those obtained by experiment, which shows the validity of the present analyses.