Abstract: The mask vibration degrades the color purity in theperfectly flat color picture tube of high resolution television. This paperis concerned with some analytical methods for calculating the natural frequency of theperfectly flat mask. Then applying the basic theories ofelasticity, we have established the governing equation for the horizontaloscillation of a thin plate and yielded the natural frequencies and modalshapes of the perfectly flat mask by modal analysis with its fouredges simply supported. The results obtained arefound to be satisfactory by comparison with experiments.It also shows that the method for determining the natural frequency of theperfectly flat mask is effective, simple and valid. The result provides a basis foroptimal designs of structure parameters in order to reducethe perfectly flat mask vibration.