We investigated the dependence of terahertz (THz) wave intensity on ex situ annealing temperature of low-temperature-grown gallium arsenide (LT-GaAs). THz waves excited by femtosecond laser pulses were emitted from photoconductive antennas (bow-tie type) formed on LT-GaAs, and the radiation intensity was measured with a Si bolometer. LT-GaAs ex situ annealed at temperatures of 700 °C and above generated 1.4 times higher radiation intensity than unannealed LT-GaAs. Transmission electron microscopy images revealed apparently improved crystal quality, explaining the observed higher carrier mobility and associated radiation intensity.