It is found by deep level transient spectroscopy (DLTS) that the oxygen thermal donor formation is suppressed in Czochralski silicon doped with donor impurities more than 1×1016cm−3. The result is explained by a new model considering electron‐hole equilibrium. The dopant concentration dependence of the thermal donor formation is analyzed, based on the model, and it is concluded that the thermal donor is a doubly charged donor.