The contrast of simple achromatic periodic patterns such as square-wave gratings is well defined and agrees with the Michelson contrast, but this is not so for chromatic contrast. It would be desirable to be able to determine the contrast of two colors for use in a general legibility metric. A method for determining the contrast of chromatic square-wave gratings, which is based on the CIE 1976 color difference formula, is proposed. In order to prove the feasibility, we first investigated the contrast sensitivity function of an achromatic grating using Michelson contrast for comparison through sets of psychophysical experiments keeping the mean luminance constant at 22.54cd/m2. Further experiments compared the curve of contrast sensitivity function for red, green, and red-green chromatic square-wave grating in terms of the proposed formula and conventional formula in LMS cone contrast space, which is frequently used in calculating the contrast of color gratings. The results revealed that proposed method is helpful in calculating the contrast of chromatic square-wave gratings and reveals important physical meaning in measuring the contrast of chromatic gratings.