Steel concrete composite columns are widely used in high rise buildings and bridges. Composite columns provide not only great reduction for the column size and weight, but also high columns efficiency and ductility. In Concrete Filled Steel Tube (CFST) columns, the steel tube provides formwork for the concrete; the concrete prevents local buckling of the steel tube. This paper illustrates a theoretical study of the behavior of CFST columns subjected to centric load by using ANSYS (18.1). The study divided into two parts. In the first part, the verification of the program was done using an experimental work of previous research, this verification showed that the capability of ANSYS program for analysis of the CFST columns with very good accuracy.  In the second part:  a parametric study was carried out to study the behavior of CFST braced, constant cross section area, centric loaded columns. The main variables were the shape of cross section (three shapes: square, rectangular and circular) and slenderness ratio (λ). The analysis shows that the increase of the slenderness ratio led to decrease the ultimate load capacity, stiffness and ductility of the CFST columns. The current ECP of concrete code underestimates the values of ultimate load capacity while, ECP of steel predicts the reasonable values for an ultimate load capacity of CFST columns.