Waterproof electric plug or receptacle for LED light string is provided. In one embodiment, the plug includes a housing comprising two prongs, two fuses, an internal space, rear passages, compartments between the space and the passages, a wall for separating the space and the compartments; an electrical assembly comprising a PCB electrically to the prongs via the fuses, and rear conductive posts in the compartments; a power cord passing the passages and having two sleeves secured to the posts, and a shell mounted around the housing. Waterproof adhesive is applied on the wall, the compartments, and front and rear ends of the PCB.