We investigated effective factors on optical density of a print sample printed by using of carbon black dispersion ink for ink jet printers, and studied characteristics of carbon black to get higher optical density of print images. We observed carefully the surface of the printed areas and we found out the relationship between the properties of the surface area and optical density. We focused the surface roughness in nanometer order, and confirmed that the optical density gets higher as the S.A.D. (= Surface Area Difference measured by A.F.M.) of the printed area becomes larger. To make high S.A.D. surface (= high surface area) of the printed areas, new furnace carbon black was developed by using our original technology. It has small particle diameter, very large DBP absorption and sharp particle size distribution. The ink jet ink containing this carbon black could make really high optical density print images even on a plain paper for copier.