In this paper, composite beams reinforced by cover plate partially attached on the lower flange are proposed in order to save steel weight. To avoid distortion raised by welding and minimize the fabrication cost, the cover plate is welded to the lower flange only at both ends and two intermediate points. Design formulae for calculating strength and elastic stiffness of such composite beams are developed based on equilibrium of stress and compatibility of deformation. Full-scale vertical loading tests are performed on three composite beam specimens. The cover plate of the first specimen is welded at both ends and two intermediate points, while that of the second one is welded continuously. The last one is a conventional composite beam without a cover plate. The test results show that the welding types give little effects on strength and elastic stiffness of the beams. The proposed formulae show good agreement with the test results.