It is very difficult to judge watermelon ripeness by external characteristics. The laser Doppler vibrometer (LDV) is a new approach to determination of fruit quality. This optical-mechanical technique was utilized for nondestructive detection of the vibration response of watermelons to predict ripeness. Finite element modeling (FEM) was used to find the optimum location for excitation and response measurement and to analyze the mode shapes. The model was considered based on red, white, and green tissues and included individual properties such as density and elasticity modulus. Modal analysis of the finite element model showed acceptable agreement between experimental results and finite element simulation. According to the mode shapes of watermelon, optimum locations for applying input vibrations and detecting output vibrations were suggested. Then watermelons were excited and their responses were recorded by LDV at the determined locations. The phase shift between input and output signals were extracted over a wide frequency range. The firmness of cut fruits was measured with penetrometer as a ripeness indicator. A regression model was developed to predict the internal texture firmness using phase shifts at statistically selected frequencies. The determination coefficients (R2) of the calibration and cross validation models were 0.9998 and 0.994 respectively. (C) 2013 Elsevier Ltd. All rights reserved.