The main purpose of this paper is to clarify the structural characteristics of steel-concrete composite columns by an experimental approach in which the steel pipe is filled with variabe concrete strength. A total of 48 specimens are tested. Important parameters in this study are the slenderness ratio of colums, the compressive strength of concrete and filled or unfilled steel pipe. Six slenderness ratios of the columns are investigated. i.e. ${lambda}$=12. 18, 25, 40, 55 and 70 and three concrete compressive strengths are varied, i.e. $F_c$=210, 300, and 400$kg/cm^2$. A concentric compressive load is applied under static loading condition without any eccentricity. Experimental results are summarized for the yield loads, the buckling loads based on the relationship between axial force (P) and non-dimensional axial displacements(${Delta}$/Lk). The superiority of the structural capacities of concrete filled steel pipe due to concrete confinement effect is quantitatively evaluated and is summarized as a function of the slenderness ratio(${lambda}$) of the columns.