Structural components are known to fail through the growth of a fatigue crack. One effective way to repair a component is by bonding fiber composite patches. In this study a pre-cracked Aluminium skin 6061-T6 of one mm thin plate is repaired by symmetrically bonding patches of fiber reinforced polymer. The repair is simulated through numerical investigations to compare the performance of Unidirectional Carbon Fiber Reinforced Polymer (UD-CFRP) patched specimen with that of Uni-directional Glass Fiber Reinforced Polymer (UD-GFRP) patched specimen for the same stiffness ratio (S) and the same fiber volume fraction ( V f ) .  The J-integral for the max and mini stress of fatigue load is calculated by finite element analysis in Ansys-15. The maximum stress intensity factor ( K max ) and minimum SIF ( K min )  are calculated from the relation between J-Integral & SIF for various crack lengths.  Then the Paris law is invoked using the relation between ΔK and the crack length (a) to determine the crack length versus fatigue cycles.  The comparison of the results of UD-CFRP and UD-GFRP specimens shows that the UD-GFRP patched specimen achieves marginally better performance. However, UD-CFRP patches were much thinner.Structural components are known to fail through the growth of a fatigue crack. One effective way to repair a component is by bonding fiber composite patches. In this study a pre-cracked Aluminium skin 6061-T6 of one mm thin plate is repaired by symmetrically bonding patches of fiber reinforced polymer. The repair is simulated through numerical investigations to compare the performance of Unidirectional Carbon Fiber Reinforced Polymer (UD-CFRP) patched specimen with that of Uni-directional Glass Fiber Reinforced Polymer (UD-GFRP) patched specimen for the same stiffness ratio (S) and the same fiber volume fraction ( V f ) .  The J-integral for the max and mini stress of fatigue load is calculated by finite element analysis in Ansys-15. The maximum stress intensity factor ( K max ) and minimum SIF ( K min )  are calculated from the relation between J-Integral & SIF for various crack lengths.  Then the Paris law is invoked using the relation between ?K and the crack length (a) to determine the crack length versus fatigue cycles.  The comparison of the results of UD-CFRP and UD-GFRP specimens shows that the UD-GFRP patched specimen achieves marginally better performance. However, UD-CFRP patches were much thinner.