PURPOSE: To improve the heat transfer rate of a corrugated fin provided with a conventional louver without increasing the size of a radiator by a method wherein recesses or protrusions or uneven surfaces are formed in the louver so that a turbulence is generated by cooling air passing the louver.

CONSTITUTION: One or two protrusions 14a are formed on a louver 24 in parallel to the longitudinal direction of a louver 14. The protrusions 14a are formed on a fin 13 simultaneously with formation of a plurality of the louvers 14 by a gear type roll. In the fin structure of a so constituted radiator core, the protrusions 14a formed, as shown by a solid 1 in arrow mark, on the louver 14 causes the generation of a turbulence at the vicinity of the louver 14 when cooling air flows. A low vortex generated by the turbulence promotes heat- exchange between a boundary layer and a main flow and as a result, a temperature gradient is also increased on the turbulence side. Thus, at the vicinity of the louver 14 where the turbulence is generated, heat efficiency is improved compared with a conventional type being a laminar flow.