A series of new monoazo yellow dyes having different alkyl substituents was synthesized to dye unmodified polypropylene fiber. Color hue of the dyes exhibited light yellow with very high chroma enough to use as yellow primary color dyes. The affinity of the dyes onto unmodified polypropylene was increased with the increase of the length of alkyl substituents. Therefore, the hexyl-substituted dye having the longest alkyl group in this experiment showed very high color strength of dyeings with K/S value of over 26 at maximum absorption wavelength. The color fastnesses to washing, rubbing and light were also improved significantly for the longer alkyl substituted dyes, so that the hexyl-substituted dye exhibited a rating higher than 4∼5 for all kinds of fastnesses.