The factors determining the pneumatization of mastoid have been disputed. Authors recently experienced a case of unilaterally excessive pneumatization of right mastoid in the patient with thymus hyperplasia which was not reported in the literatures. Studies that measure the size of mastoid pneumatization usually use a planimetric method on the lateral projection of the mastoid and recently use CT scan which is the most accurate radiographic means to assess mastoid pneumatization. The excessive pneumatization of right mastoid was animicked as a huge retroauricular mass. The CT-determined volume of right mastoid was 24.3ml, and that of left was 9.6ml, which was normally developed compairing with the average.