Effects of microstructure of molecular beam epitaxially grown GaAs films at low temperature on the properties of terahertz detection have been investigated. Microstructural changes of the films before and after in-situ annealing strongly influence the terahertz wave SNRs in the receiver. The films grown at 150degC and in-situ annealed at 600degC showed a poly- crystalline state, and those grown at 250degC or higher temperature and in-situ annealed at 600degC revealed epitaxial GaAs layers including As-rich precipitates. The SNRs of the terahertz wave were measured to be 103 to 104. The best value of the SNR 104 was obtained with the polycrystalline GaAs layer.