The bibody, a Fab-scFv fusion protein, is created by adding a scFv fragment to the C-terminus of Fab scaffold. In this class of BsAbs, such as bibody and tribody, the bispecific fragment utilizes the natural in vivo heterodimerization of the Fd fragment (the HC regions of Fab fragment) and light chain. The heterodimerization scaffold can be further incorporated with additional functions, such as scFvs, scaffold proteins, cytokines, etc. to form bivalent, bispecific molecules or trivalent, bi- or tri-specific molecules. The bibody molecules, Fab-L-scFv and Fab-H-scFv, are bispecific and bivalent. It has been shown that this format can retain the bispecific binding, a low tendency to aggregate and stable in physiological conditions. Bibody may be an option to develop bispecific fragment.