Abstract: Twenty-nine circular plain and reinforced concrete columns with different preloading stress levels are axially tested. The effects of preloading stress and sectional dimension on expansion ratio are investigated as well. The results indicate that, as the preloading stress increases, the ultimate expansion ratios of the tested columns firstly decrease and then increase. A so-called influence factor of ultimate expansion ratio of preloaded concrete columns with CFRP confinement is put forward through the regressive analysis of those ratios. This influence factor is mathematically a function of the ratio of preloading stress over confining stress, n₀ /(f_l /f_c0). Then, this non-dimensional factor is used to modify the model proposed by Teng et al for predicting the complete expansion ratio of CFRP-confined or unconfined concrete columns, thus the effect of preloading stress can be considered. The results from the modified model fit well with the test data.