The critical moment of unstrengthened and strengthened mono-symmetric and doubly symmetric steel beams was investigated using two approaches: analytical and finite element models (FEMs). The newly developed analytical model was formulated based on solving six homogeneous differential equations of a steel beam controlled by elastic lateral-torsional-buckling (LTB). The carbon-fiber-reinforced plastic (CFRP) plate was included in the analysis based on the virtual-section method. The solution consisted of a 6 × 6 eigenvalue problem. The proposed solution can predict reasonably the critical moment in the elastic range of a steel beam strengthened fully or partially at top or bottom flanges with CFRP plates. Twenty-six FEMs were built using ABAQUS software to examine the accuracy of the proposed analytical model. The FEM models included: unstrengthened and strengthened steel beams with 1-m and 2-m lengths of composite strips. Due to lack of experimental data in the elastic LTB stage and in order to further validate the analytical model, an extension of the model was proposed to compare results with other available experimental data in the inelastic-buckling zone for steel beams strengthened with steel or CFRP plates. The comparison showed that the finite element and analytical models, utilizing the virtual-section concept of CFRP plate, can reasonably predict the improvement in moment-capacity of beams in the elastic- and inelastic-buckling zones.