An increasing number of existing steel structures are nowadays at the end of their service lives and most of them are subjected to fatigue loading. To tackle the fatigue problem, carbon fiber-reinforced polymer (CFRP) composites have been proposed and successfully used as an alternative and efficient technique to strengthen fatigue prone (damaged) steel structures. Experimental studies on different CFRP strengthening systems (bonded and unbonded) showed also that using prestressed unbonded CFRP reinforcement could further enhance the performance of the strengthening system and promotes crack arrest. Different models have been proposed to investigate fatigue crack growth of CFRP-reinforced steel structures. They mainly refer to empirical damage accumulation rules (S-N curves) and fatigue crack propagation models based on fracture mechanics concepts such as Paris' law or similar. As an alternative approach in this paper, the computational assessment of Mode-I fatigue crack growth in the unreinforced and CFRP-reinforced (nonprestressed bonded and prestressed unbonded) steel plates are studied by using a cohesive zone model (CZM). The comparison between numerical and experimental results validated the finite element modelling, which will be further extended to the investigation of crack propagation under mixed mode condition.