There are three normal sizes of loading plate as 30 cm, 50 cm, 75 cm in roadbed compaction quality detection. The size effect rule for coefficient of foundation is obtained from the investigation on plate load test results by the experiment in lab. The laboratory test may control well the conditions by building the test section modeling the roadbed. The test section is 15 m by 5m. The diameters of loading plate used is 30cm, 35cm, 40cm, 45cm, 50cm, 55cm, 60cm, 65cm, 70cm and 75cm in the test. The regression equation between coefficients of foundation measured by different size of loading plate has been concluded. It is obtained that the ratio of coefficients of foundation between the plate of 50 cm and 30cm is 1.6 for granule filler, 1.65 for grind filler, and the one between the plate of 70 cm and 30cm is 2.17 for granule filler, 2.3 for grind filler.