▶ Yield strength values of Al–4Cu alloy were analysed via Hall–Petch relation. ▶ H–P analysis revealed apparently high ‘ σ 0’ (170 MPa) and a high ‘ k’ ( 0.13   MPa   m ) ▶ Precipitates and oxide particles are the likely reason for such a high values. ▶ True ‘ σ 0’ and ‘ k’ were evaluated by considering only the grain size contribution. ▶ This agrees with values taken from pure Al. Mechanical properties of bulk nanocrystalline Al–4Cu alloys with grain sizes from 47 to 105 nm, synthesized by mechanically alloying followed by vacuum hot pressing at different temperatures, were analysed through Hall–Petch relation. Hall–Petch analysis revealed a high frictional stress (170 MPa) and a high positive slope ( 0.13   MPa   m ) as compared to pure Al, which has a frictional stress (15–30 MPa) and a slope ( 0.06 – 0.09   MPa   m ). From a detailed evaluation of different strengthening mechanisms it is inferred that the Al 2Cu precipitates and oxide particles are the likely reason for such high values of frictional stress and slope.