The use of Carbon Fiber Reinforced Polymer (CFRP) in strengthening concrete and steel structures is gaining special interest as a result of low-weight to high-strength properties of the composite material. This assures effectiveness of this external strengthening as compared to other techniques. Tubular steel sections might fail prematurely as a result of local buckling due to slenderness ratio of plates, and therefore limiting axial material capacity of column. This paper presents numerical non-linear investigation of steel column externally strengthened by CFRP sheets by use of ABAQUS software. Three-Dimensional model considering material and geometrical nonlinearities is presented and verified using experimental published data. The axial behavior and local buckling of steel section are monitored considering different parameters. Slenderness ratio of steel section (h/t), and sectional aspect ratio, length/width (h/b), are the main assessed parameters in this paper. Results showed that slenderness and aspect ratios of steel section control effectiveness of using CFRP in strengthening steel columns. In addition, the use of CFRP sheets controls local buckling limit of tubular section.