In order to decrease surface defects of continuously cast slab, a newly designed innovative mold to realize uniform mild cooling has been developed. Nickel base super alloy, Alloy 718, characterized by high strength at elevated temperature and low thermal conductivity was applied as the material of mold plates instead of conventional copper. Casting experiments of medium carbon steel have been performed with pilot scale continuous caster. The following advantages of the new mold with 40% decrease of meniscus heat extraction and 75% decrease of its fluctuation were confirmed in comparison with conventional copper mold. 1) Index of longitudinal surface crack decreased by about 50%, 2) Frequency of hook (over flow type oscillation marks) appearance on which non-metallic inclusions are likely entrapped decreased by 50%, 3) Depth of "hook" decreased by 20 to 45%, and 4) Average of oscillation mark depth decreased by 35% and its fluctuation also decreased by about 50%.