Experimental　study　is　conducted　to　investigate　the　axial　compression　strength　of　66　cold-formed　thin-walled　built-up　columns　with　six　different　sections　and　five　different　lengths.　The　six　different　sections　include　C-shaped　and　U-shaped　section　for　single-limb,　and　built-up　sections　composed　by　these　C-shaped　and　U-shaped　section.　The　effect　of　slenderness　ratio,　limb　number,　and　built-up　type　on　the　axial　compression　strength　and　failure　mode　of　the　columns　is　investigated.　Based　on　test　results,　it　is　noted　that　when　the　slenderness　ratio　is　smaller　than　70,　the　open-section　columns　failed　due　to　distortional　buckling,　while　the　closed-section　columns　failed　due　to　local　buckling.　When　the　slenderness　ratio　is　larger　than　70,　the　columns　fail　due　to　global　buckling.　As　the　column　length　increases,　the　ultimate　axial　compression　strength　is　reduced.　The　slenderness　ratio　of　built-up　section　column　with　same　height　is　smaller　than　that　of　the　corresponding　single-limb　column,　the　ultimate　compression　strength　is　therefore　increased　due　to　the　failure　mode　changed　from　global　buckling　to　either　distortional　or　local　buckling.　The　experimental　results　provide　experimental　data　for　the　design　of　cold-formed　thin-walled　built-up　section　columns.　©　2017,　Progress　in　Steel　Buiding　Structures　All　right　reserved.