A slide door assembly includes a U-shaped rail box which includes two support plates in a first space so that a roller unit on a slidable door is movably on the support plates. A fixed door has one end engaged with a second space in the rail box and the slidable door is slidable relative to the fixed door. An adjustable part is mounted to one end of the rail box and includes at least one elongate slot, at least one fastening member extends through the at least one elongate slot and is fixed to the rail box so that the adjustable part is movable by releasing the at least one fastening member to fit the distance between two walls.