A method of forming solder mask, suitable for forming a solder mask on the surface of a wiring board, is provided. The surface of the wiring board includes a first region and a second region, and the surface of the wiring board has a wiring pattern thereon. The method includes forming a first sub solder mask in the first region on the surface of the wiring board by performing a screen-printing or a photolithographic process, and forming a second sub solder mask in the second region on the surface of the wiring board by performing an ink-jet printing process. The method not only improves the precision of the solder mask alignment on the wiring board and its reliability, but also increases the production rate and lowers the manufacturing cost.