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In this work, the fatigue crack growth behaviour of an aluminum alloy(6061-T6) reinforced with alumina (Al 2O3) particles is characterized. Tests areconducted in Stage I (near-threshold growth rates) and Stage II (Paris regime) at twodifferent R-ratios. Both LT and TL crack orientations are considered. The increase ofFCG rates with R-ratio has been interpreted in terms of crack closure for which, inPMMCs contributions from crack surface roughness, crack deflection or trappingmechanisms play an important role. The specimen halves are examined at the SEM afterfracture in order to highlight crack path features.