The MgB2 wires were prepared by in-situ Powder-in-Tube (PIT) method with Nb/Cu compounds tube as the sheath and then were heat treated at various temperatures for 2 h in flowing argon. The resistivity (R-T), magnetic moment (M-T), transport critical current (Ic-B) and magneto optical (MO) properties of MgB2 wires were analyzed. The results show that there is a diffusion layer between MgB2 superconducting core and Nb barrier layer when the heat-treatment temperature is higher than 750 °C, and this diffusion layer obstructs the current transfer into MgB2 superconductor core from the Cu sheath. This indicates that the best heat-treatment temperature for the MgB2 wires with Nb as the barrier layer should be lower than 750 °C