This paper presents an experimental investigation to evaluate the viability of utilizing carbon fiber reinforced polymer (CFRP) composites for strengthening of indeterminate steel beams. Ten steel I-beams were strengthened using unidirectional CFRP composites in a form of plates attached on the flanges and/or web. The beams were tested as fixedends under a one center load distributed over the piston area. The indeterminate steel beams having two fixed ends required high load to cause failure. Although, the maximum load carrying capacities were not significantly improved due to debonding of the CFRP plates, but the strengthened-beams demonstrated reasonable improvement in the flexural-stiffness and slight increase in the torsional-stiffness. In most cases, the governing buckling-mode for the strengthened and unstrengthened beams was inelastic lateral-torsional buckling combined with local flange-buckling. The findings of this study show encouraging enhancement in the structural behavior of intermediate steel beams after strengthening with CFRP composites. This study provides an important guidance for future research toward development of means to realize the full potential use of CFRP composites for strengthening of steel beams.