Radiation pattern of a parabolic antenna is calculated by means of geometrical method for diffraction. The far field in the lateral and rear directions is described in terms of the diffracted waves by the edges of the refiector in the first approximation. In the front direction the refiected wave is taken into account in addition to the diffracted waves, however, the former wave should be considered in rather different manner from the usual ray concept, because the field intensity is not uniform over the aperture plane of the reflector and therefore the ray may be spread from the limited region in the front. The parabolic reflector is regarded as the limit of many narrow ribbon plates. The field produced by this spread of the ray, which is calculated by the above consideration, becomes to small magnitude in the lateral or backward directions, therefore the field is expressed only by the diffracted waves by the edges. The computed results are compared with the experimental results obtained from a paraboloid reflector. A good agreement is obtained, particularly a valley be tween a main beam and a first sidelobe is fairly filled up.