An assembly structure of flexible board and rigid board includes a rigid board, a sub-board and a flexible board. The rigid board defines a locking gap having two side surfaces projecting toward each other to form two resisting portions. The sub-board has a standing portion inserted in the locking gap. Bilateral sides of the standing portion extend outward to form two preventing arms against a bottom surface of the rigid board. The flexible board has a base portion inserted in the locking gap of the rigid board. The base portion has a front surface and a back surface located to a front surface of the sub-board. The front surface of the base portion is against the resisting portions.