Disclosed is a fabrication method of products with closed channels by injection molding, which includes: a channel substrate mounting step of mounting a channel substrate comprising a plurality of opened micro-channels thereon to a mold; and a substrate cover molding step of molding a substrate cover by the injection molding on an upper surface of the channel substrate mounted to the mold, the substrate cover being coupled to the channel substrate while being molded. With this, the products with the closed channels can be fabricated without any additional coupling process between the channel substrate having the opened channels and the substrate cover, thereby simplifying a fabrication process and enhancing work efficiency.