An electronic device and a method of waterproofing a key thereof are provided. The electronic device includes a body, a display device disposed in the body, a key disposed in the body, and a key-waterproof structure including an insert-injected product that includes an insert-injected sealing area, and a bonding area where a silicon-not-attached portion is formed. The method includes assembling a flexible printed circuit board, to which a key is attached, through a silicon-not-attached portion of an insert-injected product, which is made by insert-injecting a bracket and silicon rubber, and bonding the silicon-not-attached portion.