Concrete filled steel tube (CFST) columns are composite member mainly consists of concrete infilled in steel tube. In current construction industry, CFST columns are preferred to provide lateral resistance in both unbraced and braced building structures. In this paper, finite element studies were carried out on concrete filled steel tube columns under an axial composite loading by using ABAQUS/CAE. The inelastic behavior of concrete and steel tube was defined to the model by using concrete damaged plasticity model (CDP) and Johnson-cook model respectively which is available in ABAQUS/CAE. The diameters of columns were considered as 100 mm, 125 mm and 150 mm, whereas the length of columns was kept constant, i.e. 600 mm for all models. The thickness of steel tube was considered as 4 mm and 5 mm for all diameters of columns. The concrete infilled of grade M30 was used in this study. The simulations were carried out against composite loading to study the response of CFST columns in terms of load carrying capacity, displacement and von-mises stresses. The mesh conversion study was also carried out to obtain the best size of mesh corresponding to the experimental load carrying capacity of CFST columns