This paper shows the results of R&D program carried out to optimize NbTi wires for the Poloidal Field (PF) coils of ITER. The influence of heat treatment on the microstructure was investigated by using TEM to find the relations between the microstructure and the critical current densities at 4.2 K for NbTi superconducting composite wire. Typical microstructure (in transverse cross-section) shows α-Ti pinning centers dispersed within the superconducting β-NbTi matrix with strong fold and curl for the NbTi wire with high critical current density at 4.2 K. It is believed that the higher temperature is beneficial to a higher Jc value at 4.2 K, but the Jc values slightly decrease with increasing the heat treatment times