Following the previous studies regarding blue and yellow dyes, a series of new red dyes having different length of alkyl substituents on the same chromophore were synthesized in order to dye unmodified polypropylene fiber. The affinity of the dyes onto unmodified polypropylene fiber was increased with the increase of the length of alkyl substituents. Therefore, the longest hexyl-substituted dye showed very deep shade of dyeing with K/S value of around 30 at maximum absorption wavelength. Within the range below 2 % o.w.f., the exhaustion (%) showed more than 80 %. The color fastnesses to washing, rubbing, and light of the dyeings were also improved greater for the dyes having longer alkyl substituents than the shorter ones. Since color hue of the dyes exhibited very strong red, they could be considered to be used as the primary red color dyes for unmodified polypropylene fibers.