The characteristics and annealing behavior of vacancy clusters in neutron transmutation doped silicon (NTDSi) were investigated by means of Fourier transform infrared spectroscopy (FTIR) and photoinduced transient spectroscopy. FTIR results showed that three IR absorption peaks at 707, 742, and 776 cm-1 occurred during annealing NTDSi samples at temperatures ranging from 350 to 600℃. By comparing the present results with those reported earlier, the defect center (we named it the P0 center) which gives rise to these IR absorption bands, was assumed to be a multivacancy cluster consisting of at least six vacancies. Photoinduced transient spectroscopy results showed that a P0 center introduced two acceptor levels at EC-0.427 eV and EC-0.524 eV inside the energy gap.