Doppler-broading spectrum of positron annihilation radiation have been measured by using 2-detector coincidence technique for pure metals of Si, Ni, Ti and NiTi alloys with five different chemical compositions. The W parameters of NiTi alloys are calculated. The experimental results show that some 3d electrons are localized to form covalent bonds when Ni and Ti atoms aggregate to form NiTi alloy, decreasing the free electrons in metal bonds. The covalent bonds and metal bonds coexist in NiTi alloy. The Ni-49at%Ti alloy has the biggest W parameter, indicating that the quantity of defects is least in this component alloy. This may be the reason why the Ni-49at%Ti alloy has the best shape memory effect.