An attachment assembly for a personal digital assistant (PDA) includes a first cover portion, a second cover portion and an attachment unit. At a front surface of the first cover portion is formed a plurality of fastening holes to adequately locate attachment arms for holding the PDA. The attachment unit includes an attachment body. A projection piece, an opened ring, a rod and a stopper piece are formed at a first side of the attachment body to mount with to a gear, a lock, a pressing button, a fastening frame, a first short resilient member and a second short resilient member. A slot is formed at a second side of the attachment body to receive the fastening frame and the long resilient member. By pressing down the pressing button, the lock, the gear and the long resilient member interact with one another to achieve the attachment of the PDA.