Disclosed are foldable bed frames, and oblique supports and connecting mechanisms for foldable bed frames. A foldable bed frame includes first and second sub-frames, and first and second side leg assemblies. The first and second sub-frames, each having top and bottom surfaces, are pivotally connected to each other. The first and second side leg assemblies are pivotally connected to the sub-frames and positioned within the space between the top and bottom surfaces of the sub-frames when folded. In some cases, the foldable bed frame includes a middle leg assembly, oblique supports, connecting pieces, and/or support connecting mechanisms.
