It is well known that, in a color CRT, color change occurs due to the magnetization of a shadow mask and frame under the enviromental magnetic field. Small phosphor dots are provided to give the margin between the neighbour dots for the beam spot shift by the magnetic field. However, as a result, the screen brightnessis sacrificed. In order to clarify the influence of the magnetization of a shadow mask and frame quantitatively, we measured the magnetic field distribution around the mask and frame, and calculated the electron beam trajectory change with a slit model to simulate a shadow mask. The results teach us that the big change of the trajectory and velocity occur in the slit region with the high magnetic flux density.