An electron gun for a cathode ray tube includes a triode having cathodes, a first grid and a second grid, one or more third grids through which electron beams emitted from the cathode pass, a fourth grid opposing the third grids and forming a main focus lens with the third grid, a shield cup connected to the fourth grid and supplying a high voltage to the fourth grid, and a correction grid disposed between the fourth grid and the shield cup and having R, G, and B beam through holes arranged along a line. The R and B beam through holes have openings with asymmetrical shapes that are symmetrical with respect to each other and relative to the straight line, balancing astigmatisms and improving resolution.