Abstract: The theoretical calculation methods,i. e. , the numerical method and the simplified integral method, for predicting particle flow rate through an orifice have been established by using the theory of nonfluidized gas - particle two phase flow. The active stress state and the passive stress state of the particle flow are discussed. The choice of the signs ±and in the theoretical equations is also proposed. The empirical equations for evaluating the correctness factor,Co,which is used to make correction for the particle flow rate Gp calculated by the numerical method and the simplified integral method under the passive stress state, are also given on the basis of regression analysis for experimental data. The predicted value of particle flow rate is in good agreement with the experimental data.