In recombinant strains, many proteins and enzymes are expressed as inactive and insoluble inclusion bodies. For soluble expression of an active form of StyB, an NADH-flavin oxidoreductase, several recombinant Escherichia coli strains were developed and tested. Among them, strain BL21(DE3)pLysS effectively produced an active and soluble form of StyB as about 9% of the total protein content, when cultivated at $20^{circ}C$ with 0.5 mM IPTG. The solubly expressed StyB has the highest oxidoreductase activity at pH 6.5-7.5 and $37^{circ}C$. Substrate dependence profiles of the StyB-catalyzed reaction showed that the maximum specific activity($V_m$) and half saturation constant($K_m$) were $1,867{pm}148;U/mg$ protein and $51.6{pm}11{mu}M$ for NADH, and $1,274{pm}34;U/mg$ protein and $8.2{pm}1.2{mu}M$ for FAD, respectively. This indicates that solubly produced StyB has 6- to 9-fold higher oxidoreductase activities than the in vitro refolded StyB from inclusion bodies.