Abstract: Field loading tests on six large-diameter post-grouting bored cast-in-situ piles are conducted to study the size effect of post-grouting piles and the influence of grouting on them. The results show that the influence of size effect on skin friction decreases due to the solidification and compaction of the shaft side mud after grouting. Whereas the size effect of skin friction can be found obviously for the higher layers unaffected by the grouting. The size effect coefficient of the measured skin friction is smaller than the calculated one for the pile with a diameter of 900 mm and 1000 mm and decreases with the increase of the diameter. It is less conservative to calculate the size effect coefficient with the method used in the current code. It is also found that the size effect coefficient for the sand or gravel layers is smaller than that for clay or silt soil layers. In addition, the size effect of tip resistance of the post-grouting piles can be eliminated.