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Summary form only given. We present the observation of coherent acoustic phonon lasing behavior in InGaN/GaN MQWs. InGaN/GaN MQWs behaved as a phononic-crystal distributed-feedback cavity, thus only the specific phonon modes with wavelengths matching the MQW period width could be selected for coherent output. By using femtosecond ultraviolet pulses as pumping sources, coherent acoustic phonon amplification with over-10 dB acoustic gain was measured. When the induced acoustic gain is higher than the acoustic loss due to its acoustic traveling nature, a clear laser-like threshold behavior was observed.