A synthetic fiber having an uneven surface structure consisting of wrinkles having ridges and recesses of the surface of the fiber and not presenting specular luster, which may be used, for example, as artificial hair for wigs, can be produced easily and stably by melt-spinning a starting synthetic resin such as nylon and passing the spun monofilament through a cooling bath at a temperature not lower than 30°C for a period of time sufficient for developing wrinkles in its surface. Inclusion of a pigment such as carbon black in an appropriate amount in the starting material is effective in providing dense distribution of wrinkles.