Carbon fiber reinforced polymers (CFRP) to repair and strengthen the steel I-beam has been increasingly used since last decade. CFRP composites bonded to steel members offer many advantages over steel plate bonding including excellent corrosion resistance, high stiffness and high strength to weight ratios etc. This study numerically investigates the fatigue performance of CFRP strengthened steel I-beam. One non-strengthened control beam and several strengthened beams using steel plates and CFRP strips were investigated primarily. The effect of slenderness ratios of web on fatigue behavior of CFRP strengthened steel I-beam is investigated. The beams were simulated in full three-dimension and fatigue life was investigated by using general-purpose finite element program, ANSYS. Simply supported beam subjected to two loads on compression flange is analyzed to show the effect of CFRP and slenderness ratios on fatigue behavior of steel I beam. The results show, that the life cycle of a CFRP strengthened beam before failure is higher than that of bare beam. It is also observed that beams with higher slenderness ratios (fixed thickness of the web) give better fatigue performance.