A frame structure includes two side frames and a foldable frame coupled with the side frames to fold between an unfolded position and a folded position. The foldable frame includes a transmission link supported between the side frames, a sliding joint slidably coupling with the transmission link, and two folding arms coupling the transmission link to the side frames respectively. Each of the folding arms has an upper end pivotally coupling with the sliding joint and a lower end pivotally coupling with the respective side frame. At the unfolded position, the sliding joint is upwardly slid to an upper end of the transmission link to move the side frames away from each other. At the folded position, the sliding joint is downwardly slid away from the upper end of said transmission link to move the side frames towards each other for folding the side frames side-by-side.