With the improvement of radiodiagnostic technic, surgical microscope and microsurgical instrument, the acoustic neuroma can be diagnosed early and treated without morbidity. On previous report, we have reported the measurement of the internal auditory meatus with impression meterial on dry skull. Here, we report the result of radiological measurement of internal auditory meatus with Stenver's and Caldwell's projections as follows ; 1) The mean length of the internal auditory meatus on Caldwell's projection is 14.4 mm on the right and 15.3 mm on the left and on Stenver's projection it is 12.1 mm on the right and 11.3 mm on the left. The mean side difference on Caldwell's projection is 0.9 mm and it is 0.8 mm on Stenver's projection. 2) The mean diameter of the internal auditory meatus on Caldwell's projection is 5.4 mm on the right and 5.3 mm on the left and it is 6.3 mm on the right and 6.5 mm on the left on Stenver's projection. The distribution of side difference on caldwell's projection is less than 1.0 mm in 90% and there is not even a case which is larger than 2.0 mm. 3) The shape of the internal auditory meatus is symmetric in 82% and the straight form is most common with the incidence of 75.7% and then narrow medially form 19.3% and oval form 5.1%. 4) With above data, it can be concluded that, for study of internal auditory meatus, Caldwell's projection is more desirable because it has less distorsion and advantage of bilateral comparision in one projection.