Abstract： Using (20-300K) Hall coefficient measurement and (6-300K) IR optical absorption, thermal donor generated in P-CZ Si single crystal after heat treatment at 450℃ over a wide range of time has been studied. It is shown from Hall coefficient measurements that two donor levels in the specimen after 100 hrs. annealing are observed: E1= 56 meV, E2= 110 meV. Low temperature IR optical absorption of specimens after annealing of different duration shows complex structure arising from thermal donor. The intensity and number of the absorption peaks increase with increasing duration of annealing. The results obtained can be phenomenologically explained fairly well by the hydrogen-like effective mass approximation theroy assuming the occurence of different species of doubly charged donor. The oxygen-related thermal donor can also be possibly understood as doubly charged donor. The number of species of thermal donor is closely correlated to the duration of annealing.