Single-chain Diabody (scDb) is a derivative of Diabody. In Diabody, two polypeptide chains are assembled noncovalent. scDb connects the first chain and the second chain of diabodies by a flexible linker, transforming the heterodimeric format of diabody into a single-chain format. In scDb, all four variable domains are on one chain and assemble into diabody-like molecule. The middle linker can force correct assembly and improve stability. scDb can also be fused with Fc or CH3 domain to exert Fc-mediated functions and/or extend half-life.