The behavior of cold-formed steel (CFS) built-up members is fascinating, and owing to its advantages, the necessity of using built-up members is increasing in construction practice. However, as per the current design specifications of the American Iron and Steel Institute (AISI), there is no explicit design method for CFS built-up members. The objective of this investigation is therefore to examine the appropriateness of using the current AISI design method for CFS open or single cross-section members toward the design of built-up members with slender unstiffened flange elements subjected to bending. The investigation results indicate that the previously suggested design procedure (built-up elastic buckling model with double flange element) results in unconservative design predictions compared to the experimental results (MEXP versus MDSM). The reason for the unconservative design prediction by AISI expression with the previously suggested procedure is due to the overestimation of the critical elastic local buckling stress and incorrect failure mode prediction of the built-up cross section, which is a key input to the direct strength method (DSM). Hence, modified design equations and procedures are suggested for the CFS built-up beams with slender unstiffened flange elements. The modified design procedure is formulated from the investigation of failure modes. It is also shown that the newly proposed design equation with the suggested procedure for determining the local buckling stress for slender unstiffened flange elements is reliable. © 2021 American Society of Civil Engineers.