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The effect of Si additions on microstructure and mechanical properties of Al–4Cu–1.3Mg (wt%) alloy was investigated by microhardness measurements, TEM and HRTEM observations, mechanical properties test. The results indicate that the cubic σ-Al5Cu6Mg2 phases precipitate from matrix of the Al–4Cu–1.3Mg–0.3Si and Al–4Cu–1.3Mg–0.6Si alloys. The presence of σ-Al5Cu6Mg2 phases improved the tensile strengths of the alloys efficiently at elevated temperatures. The Al–4Cu–1.3Mg–0.9Si alloy following T6 peak ageing temper contained S′-Al2CuMg and β′-Mg2Si precipitates but slightly inhibited σ-Al5Cu6Mg2. The absence of σ precipitates leads to a loss of the tensile strengths of the alloy at elevated temperature.