Abstract: The behavior of FRP-confined concrete square columns under uniaxial compression is analyzed by 3-D nonlinear finite element method. Based on the analysis result, the columns section stress distribution and mechanism are discussed and pointed out that there is shear stress and shear strain in the columns cross section which is under uniaxial compression. It is also explained the cause of FRP stress asymmetrical distribution around the cross section. A suggestion of dividing weakly confined region from cross section by the ratio of the first principal stress and the third principal stress is developed. It is also discussed the affection of fillet radius of cross section on stress distribution.