In order to research the human color vision characteristics, the color discrimination threshold experiment at the five basic CIE color centers was carried out by a panel of 3 observers with normal color vision on the CIE1976 a*-b* plane using CRT-generated color stimuli based on the psychophysical method of interleaved staircase. The observer test accuracy was estimated using performance factor PF/3, and the color discrimination thresholds were represented via chromaticity ellipses. The detailed processing and analysis of the experimental results indicated that the color discrimination characteristics of human eyes were different for individual color regions and color directions in the iso-lightness a*-b* plane, and namely the CIE1976 a*-b* plane is visually non-uniform. The visual color-difference scale in the red-green direction was generally smaller than that in the yellow-blue one for the five CIE color centers. The experimental data could provide references for the improvement of uniform color space and color-difference formula.