The purpose of this study is to analyze the axial load capacity equations for rectangular CFT columns in the AISC 360-10 (the US design code for structural steel buildings) which is being investigated as a revision of Korean Building Code. Main parameters were the width-to-thickness ratios of composite member and material strength as 30~100MPa of concrete compressive strength and 325~725MPa of yield strength of structural steel. The first method of changing width-to-thickness ratios was to fix the thickness of structural steel to 5mm and to increase the size of member. The second method was to fix the size of member to $620mm{ imes}620mm$ and to change the thickness of structural steel. The analysis results indicated that axial load capacity used higher strength steel is lower than the capacity used lower strength one in some regions although the section size was equal. The reason of this result is due to the different section type that is classified by the equation of limit width-to-thickness ratios. When the yield strength of structural steel become higher, its section type and axial load capacity equation is changed though its width-to-thickness ratio is the same with the lower yield strength members. The region of this phenomenon is become wider following to the increase of concrete compressive strength because part of concrete load capacity is govern the total load capacity of members. To resolve this irregular phenomenon that axial load capacity is reversed in some region, it is necessary to modify the code provision for calculation the load capacity in high strength materials.