In the current study, a new strut effectiveness factor based on Strut-and-Tie Model (STM) was proposed to assess the ultimate shear strength of carbon fiber reinforced polymer (CFRP) strengthened concrete deep beam. Derivation of new effectiveness factor of concrete struts is based on the failure principles of Mohr–Coulomb. Two types of concrete failure mode, namely diagonal splitting and concrete crushing are proposed and investigated. Experimental results obtained from the literature results were evaluated using the proposed effectiveness factor and compared with other existing models. The proposed effectiveness factor is validated by utilizing the experimental results which shows good accuracy for predicting the shear strength of CFRP-strengthened concrete deep beams.