A new analysis method for directly observing deep levels based on photo Hall transient measurement was developed,in which Schottky or p n junction or MIS structure is not necessary to be made. The measured parameters of defects were obtained at almost zero electric field. This method overcomes the influence of electric field, Debye tail,etc.on the characteristics of defects, so it can compensate the insufficiency of deep level transient spectroscopy.The capture barrier of DX center in Ga 0.7 Al 0.3 As was measured by using this method.