An experimental investigation on Cold-formed steel built-up sections with various slenderness were conducted under compression loading. The CFS built-up sections were fabricated by connecting C channels using self-drilling fasteners. A total of 18 compression tests were carried out which includes five different intermediate connector spacings and three different lengths. The ends of the CFS built-up columns were welded to the flat plate to arrest any rotation at the ends thereby simulating fixed-fixed support condition. The experimental results observed indicate different buckling modes such as global and interaction of global and local buckling modes. The experimental test results were compared with the North American Specifications (AISI 2016) for Cold-formed steel design. The comparison of test design results indicates that the design results are unconservative for many specimens. The reliability analysis indicates that the current design procedure of AISI (2016) is unreliable for determination of the cold-formed steel built-up columns. Copyright © 2018 by The Hong Kong Institute of Steel Construction.