A simulation program of CRT design, specially focused on mask design, is proposed in the present study. Incorporating the back-tracing results oh exposure system along with three-dimensional spatial geometric relations, the weighted least square method is applied to solve the characteristics of each R-G-B trio in full screen. The nesting arrangement for full screen is obtained by setting a flat mask pattern, included as horizontal/vertical pitches and aperture size. in the same manner, the landing distribution of electron beam could be found by inputting the magnitude of misregistrations. The improvement of mask design is judged according to the increase in the guard-band of nesting distribution. Graphic User Interface (GUI) is another target in this work. Within GUI environment, design engineers can comfortably operate the simulation system to input mentioned design parameters, such as: nar mask pattern:, formed mask geometry, q spacing, etc. Also, the simulation results, both R-G-B frio and nesting arrangement, can be shown graphically.