The amplitude of terahertz radiation (THz) from a series of oxide films on GaAs was measured by time resolved THz emission system. The barrier heights and the densities of the interfacial states are determined from the PR intensity as a function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low interfacial state densities in the range of 1011 cm-2. It is found that the amplitude of THz radiation from Al2O3-, Ga2O 3-, and Ga2O3(Gd2O 3)-GaAs structures are increases with interfacial electric field. The reason is that the electric field is lower than the "critical electric field", the amplitude is proportional to the product of the electric field and the number of photo-excited carriers. However, as the field higher than the critical electric field, sample of air-GaAs structure, the lower THz amplitude was obtained due to the maximum drift velocity declines slightly as the field increases.