The filed shape must be frequently treated in terms of shielding block choices in man clinical applications of electron beam therapy. Because of the difficult of measuring the output factors for individual rectangular electron beam field for each patient, it is necessary to know the output factors for rectangular field size electron beam fields to accurately deliver the prescribed dose to the target. The output factors for rectangular fields have been found that the equivalent area technique generally used in X-ray treatment can be utilized reasonably well to predict the output factors for rectangular electron fiel