Thin panels of aluminium alloy 6061-T6 with a centre pre-crack were repaired with a patch of unidirectional CFRP plies applied symmetrically. Dimensions of the patch were varied to explore appropriate patch size to check the growth of the crack under a tension–tension fatigue load in Mode I. The criterion employed was that the difference in stress intensity factor, ΔK, corresponding to maximum and minimum stress of the fatigue load, should be smaller than the threshold value of ΔK. Necessary minimum length, width and thickness of the patch were found for a crack length and a fatigue load. The investigation on the variation in the length of the pre-crack showed that repair through symmetrically applied patches was quite effective even for long-length pre-cracks. The symmetrical patching was also quite effective for higher fatigue loads; appropriate patch thickness was explored for varying fatigue loads.