Generally, two basic routes called as in-situ and ex-situ process are used for composite MgB2 wires now. Mentioned processes have been used for making of single-core composite wires. The applied heat treatment parameters influence the microstructure of MgB2 phase, critical temperature and critical current density, but it has also decisive effect on the MgB2/metal reaction. It was found that the transport current densities are much more sensitive to the used sheath material than critical temperatures. The main factors limiting the transport current density are cracks introduced by deformation and porosity caused by the boron diffusion in ex-situ and in-situ wires, respectively.