Abstarct: Every moment new materials are introduced to the world. In developed countries composites are extensively used as the main materials for tall buildings, bridges and so on. Concrete Filled Steel Tube (CFST) columns are utilized as one of the most important composites. Therefore, understanding the behavior of these columns is essential for civil engineers. In the present study, the behavior of CFST columns under compression loading is investigated. A CFST column consists of 2 parts: a concrete core and a steel tube. The concrete core is confined by the steel tube. In the first stage, experimental equations acquired by various researchers were gathered for modelling of concrete core. Afterwards, the most accurate experimental equation which was compatible with 13 different types of confined concrete was defined by comparing the experimental and analytical curves. Furthermore, equations of steel curve were collected and simplified to be used in the parametric studies. Then, concrete, steel tubes and their interface were simulated by Concrete Damage Plasticity (CDP) or Drucker-Prager (DP), elastic–plastic and surface to surface or general contact models in ABAQUS, respectively. One of the most prominent models investigated in the current study was related to Fiber Reinforced Self-Consolidating CFST (FRSCCFST). Other investigations were also conducted in this study including parametric studies on mechanical and geometrical variables, proposing eight procedures for strengthening of square CFST and circular FRSCCFST columns, considering eccentric load, geometrical imperfection and residual stress of steel tube. baxsed on the results of this research, it was concluded that all 13 models can predict the ultimate strength of CFST columns with 90% of accuracy. Moreover, the behavior of curves before and after the peak load was predicted with reasonable accuracy for all columns. It was also concluded that the model proposed for FRSCCFST columns can predict their behavior by changing the fiber volume content. Failure modes of all models were almost the same as those acquired baxse on experimental specimens. It was also shown that the effects of geometrical imperfections and residual stress are negligible. Investigations on the strengthening methods for CFST columns showed that utilizing steel circular stirrups in circular CFST columns as well as using the oblique rib stiffeners in square columns CFST is the best method for improving their axial compressive strength.