A new method is proposed to determine the self-admittance of a slot in a broad wall of the waveguide. By considering the effect of mutual coupling of the slot array,the method transforms the self - admittance measurement into the test of the reflection or transmission coefficient of the slot array and the self-admittance of the single slot is calculated by means of the Newton's iterative method. The result obtained by the present method is in good agreement with that given by Hamadallah.The project supported by the National Naatural Science jFoundation of China and the Foundation of State Educat Commission