We developed the tension mask of which thickness increased from 25 urn to 50 urn to reduce the mask cost and improve the vibration property. First of all, the limitation by which rail structure can support is calculated and the optimal thickness of mask is determined. To prevent the reduction of brightness and brightness uniformity, the dimensions of mask was reassigned. As a result, the increase of mask thickness brought about a reduction in cost and improvement of howling property which had been a weak point of flat CRTs.