This paper presents the flexural behavior and strengthening effect of precast reinforced high-strength concrete T-beams strengthened using CFRP sheets with different existing steel reinforcement ratios and CFRP strengthening levels. In general, normally strengthened beams failed by interfacial shear failure (delamination) within concrete, instead of by tensile failure of the CFRP sheets. The delamination occurred suddenly and the concrete cover that was cracked vertically by flexure was spalled off due to the impact energy. The strengthened beams were stiffer than the control beam before and after yielding of the reinforcement. The ultimate load considerably increased with an increase in the number of CFRP layers, while the ultimate deflection significantly decreased. The tensile force of the CFRP sheets and the average shear stress of concrete at delamination failure were linearly proportional to the CFRP strengthening level. Therefore, the increment of the ultimate load obtained by strengthening was linearly proportional to the strengthening level. On the other hand, the steel reinforcement ratio does not seem to have a significant effect on the increment of the ultimate load.