Abstract: A quadratic function model of plate damage by projectile impact, as related with its diameter and velocity is presented, whose positive value means that the impacted plate is penetrated and whose negative value means that the impacted plate is not penetrated. A method to determine the coefficients of the quadratic function using test data is proposed. The penetration criterion of impacted plate based on projectile's diameter and its velocity can be obtained by letting the quadratic function to be zero, and a curve shape identification method for the penetration criterion is also given in the paper. The procedure to determine the penetration criterion is shown through a group of simulated data, and the feasibility of the method is also confirmed by the example.