We herein present comparative investigation on the dielectric properties of both ceramic and single crystal SrTiO3 samples in the temperature from room temperature to 1073 K. Two relaxations were observed in both samples. They behave as Debye-like and relaxor-like relaxations in ceramic and single crystal samples, respectively. These relaxations were found to be bulk effect related to oxygen-vacancy. In single crystal sample, the relaxations result from the long-range conduction associated with singly and doubly charged oxygen vacancies. In ceramic sample, the oxygen vacancies are more strongly localized in relation to the crystal. This leads to a new phenomenon of formation and dissociation of oxygen vacancy clusters before the vacancies make contribution to the long-range conduction. The low-temperature relaxation in ceramic sample was determined by the clustering and dissociating processes of the oxygen vacancies. The high-temperature relaxation in ceramic sample was found to share the same mechanism as that in the single crystal sample.