A lockable door assembly of an electronic device is used to cover and seal an opening of a housing of the electronic device. The lockable door assembly includes a door, a first sliding member, and a second sliding member. The door is pivoted to the housing to cover the opening. The first sliding member is movably disposed on the door for being engaged with a rib formed on the housing, so as to stop the door from rotating. The second sliding member is movably disposed on the door, and the sliding paths of the second sliding member and the first sliding member are intersected. The second sliding member is used for being inserted into a nick of the first sliding member to prevent the first sliding member from moving, thereby a double secure mechanism is achieved and the thickness of the lockable door is reduced.