The present invention relates to a case assembly structure. The case assembly structure includes a first case and a second case. The first case has a first connection portion including a first protrusion, a first trench and a second protrusion from an external side to an internal side thereof. The second case has a second connection portion including a third protrusion and a second trench from an external side to an internal side thereof. The first trench and the second protrusion of the first connection portion of the first case are corresponding to the third protrusion and the second trench of the second connection portion of the second case, and an inner wall of the first protrusion of the first connection portion of the first case is in contact with the external side of the second connection portion of the second case such that the first case and the second case are combined together.