In this paper the design strength of lean duplex stainless steel material rectangular CFDST axial slender columns and slender beam-columns was investigated. Fully-effective (Feff), slender (S) and very slender (VS) cross-sections of the outer tubes were considered. The ultimate axial strengths of axial slender columns were compared with the design strength predictions by Eurocode 4 and AISC specifications for the CFST columns. It was found that, strength predictions according to Eurocode 4 and the AISC specification provided conservative results for only short columns, while they gave unconservative results for slender columns. A modification was made in the reduction factor ( χmod ) of Eurocode 4 to predict the axial strength of current models. On the other hand, the interaction relationships of beam-columns according to Eurocode 4 and Tao and Han (2006) were compared with the finite element results. It was concluded that, the two methods were suitable for only the short beam-columns. Hence, a modified interaction moment-axial force equations based on Tao and Han (2006) was proposed to predict the results of the finite element.