Monomethylhydrazine (MMHy) is examined as a nitrogen source material for C-BE growth of nitride semiconductors on GaAs(0 0 1) and (1 1 1)B substrates. We find that the AlN buffer layer insertion is effective in obtaining a hexagonal GaN layer with high crystallinity on the GaAs(1 1 1)B substrate. From the mass spectrometric analysis during MMHy exposure to the GaAs and AIN surfaces, it is clarified that the effectiveness of the AIN buffer layer is due to the high reactivity of AIN surface to the MMHy decomposition.