The internal quality of watermelon, maturity and cavity existence, can be detected by traditional expert farmers who has more than 10 years experience. They tapped the watermelon by hand and listened the produced sound from watermelon to judge their qualities. This study is to evaluate the internal quality of watermelon, maturity, cavity existence and orientation, using a nondestructive technology-acoustics. An impulse vibration was produced with hammer to apply to the watermelon. The resonant frequency of watermelon after striking was found from acquired signal. The ripeness index was derived from the elastic sphere model to determine elastic modulus. From experimental results, there is a correlation between the maturity of watermelon and ripeness index. The cavity existence and orientation of watermelon can be evaluated by the resonant frequency of vibrational signal of watermelon through FFT. This technology of acoustics to evaluate the cavity existence is more reliable and accurate than the expert farmer.