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For the first time, high frequency (~900MHz) lateral-mode thin-film piezoelectric-on-diamond resonators are reported with insertion loss values as low as 2.6dB when terminated by 50Ω impedance. In this work, the surface of the as-deposited ultrananocrystalline diamond (UNCD) is polished to promote the direct growth of highly oriented c-plane AlN which is critical in enhancing the electromechanical coupling and therefore reducing the motional impedance of the resonator. The frequency of resonators fabricated on 3-4um UNCD with a 930GPa Young's modulus is measured to be about 2× higher than similar devices fabricated on 5um of silicon. These resonators exhibit a low temperature coefficient of frequency (TCF) of -9.6 ppm/°C and have a great potential for being utilized in channel-select high frequency filters.