The chloride migration coefficients of concrete specimens with different shapes and different sizes were tested with modified rapid chloride migration test. The results indicate that the shape and thickness of concrete specimens slightly affect the test results of chloride migration coefficient of concrete specimens; When the size of the diffusion face of the specimen is larger than the electrode plates of the cell, the measured chloride migration coefficient will be a little smaller. The thickness, thus the electrode space also has effect on the test results, the thicker the specimen, the smaller the chloride migration coefficient measured. The effect is larger for that the specimen with the migration face larger than the electrode plates of the cell than those about the same size with the electrode plates