The authors demonstrate the growth of polycrystalline GaAs thin films on polycrystalline chemical vapor deposition (CVD) diamond by low-temperature molecular beam epitaxy. The low-temperature GaAs (LT-GaAs) layer is easily polished compared to the CVD diamond, and this process results in a reduction of rms surface roughness from >50 to <5nm. This makes the LT-GaAs on diamond layer an ideal wafer-bonding interface for high-power semiconductor devices. The samples were grown at 0.2μm/h with a substrate temperature of 250 °C and a 1:8 III/V beam equivalent pressure ratio. The samples were analyzed by x-ray powder diffraction, atomic force microscopy for surface roughness, and in situ reflective high-energy electron diffraction during molecular beam epitaxy growth. The authors also measure the thermal conductivity of the GaAs layer on CVD diamond using pump-probe time domain thermoreflectance.