Although externally bonded carbon fiber-reinforced polymer (CFRP) sheets have been used more commonly for the strengthening of existing reinforced concrete (RC) structures, the effects of this strengthening method on prestressed concrete (PC) beams have not been well clarified. In this study, an experiment was conducted to investigate the behaviors of pre-tensioned PC beams without shear reinforcement strengthened in shear using externally bonded CFRP sheets with the consideration of CFRP reinforcement ratio, stiffness and prestressing level in strands. The results implied that the shear resistance in the strengthened PC beams depended on maintaining the combination of beam action, arch action, and the bonded sheets. The failures and the increase in shear capacity of the strengthened PC beams were strongly affected by the amount, thickness of the CFRP sheets, and the effective prestress in the strands. The higher effectiveness of strengthening could be obtained in the beams strengthened with a higher CFRP reinforcement ratio of the same thickness, higher elastic modulus sheets or having higher prestressing level. Nevertheless, the increase in the thickness of the sheets did not provide a better performance. Moreover, the high inaccuracy in the calculations using the equations stipulated in the recent design guidelines compared to the increment of the shear strength obtained by the experiment was explained by the shear-resisting mechanisms.