A new series of anthraquinoid magenta dyes having alkylphenyl substituents was synthesized and applied toward pure polypropylene fibers. The affinity of the dyes toward polypropylene fibers was increased gradually with the increase of the length of alkyl substituents. The optimum length of alkyl group was determined as heptyl substituents from the practical point of view. The color values of the dyes on polypropylene fabrics displayed slight bluish red, namely magenta which is more practical than primary red in color matching. The good fastness ratings to washing, rubbing and light were obtained for the dyes having longer alkyl groups than hexyl substituents. Expecially they exhibited ratings 4 of light fastness, which is higher than primary red dyes previously reported.