Abstract: In order to detect the size and depth of an internal deep flaw in materials,combining out-of-plane displacement digital speckle pattern interferometry(DSPI) with elasticity theory,the out-of-plane displacement distribution of a plate with an internal flaw was analyzed.Then it was verified by numerical simulation and DSPI.The out-of-plane displacement curves and its first-order derivative curves obtained by the above three methods were compared with each other.It was shown that results obtained from these three methods were in good agreement.The experimental error was analyzed.Finally,the flaw size was calculated from the first-order derivate of the out-of-plane displacement.Both the errors of the experimental and theoretical results were in the acceptable range.The results show that DSPI is suit for detection of internal deep flaws with high accuracy.