Human extracellular superoxide dismutase(hEC-SOD) is a secreted tetrameric protein involved in the protection of a human body from oxygen free radicals. Its three-dimensional structure has not been confirmed. hEC-SOD couldn′t be expressed in E.coli. We constructed a hybrid enzyme, which comprises the N-terminal and C-terminal domains from hEC-SOD, fused it to human Cu,Zn-SOD. The hybrid enzyme is expressed successfully in E.coli. Further, we analyzed the expression of hEC-SOD in E.coli by mRNA differential displaying.