Disulfide bond formation protein A, DsbA, is one of the important proteins located in E. coli periplasm, which is a foldase facilitating the folding of nascent secreted proteins, especially for those with many pairs of disulfide bonds. The crystal structure and phylogenetic analysis of DsbA and DsbA-mediated protein folding, alternatively in vivo and in vitro, are summarized. Both the extremely low pK_a of Cys^{30, about 3.5, and the destabilizing effect of the active site disulfide contribute to its strong oxidizing power. The Cys30is also considered as the most important residue closely related to its activity using site-directed mutagenesis methodology. DsbA could effectively assist proteins folding, both in vivo coexpressed with the target protein, and in vivo replenished as foldases. Moreover, DsbA also has the chaperone-like activity in the assistant refolding of genetically engineered inclusion bodies.}