<p>The grain boundary (GB) plays an important role in dislocation motion during deformation. The aim of this work was to reveal the effects of GB precipitation on the strengthening and GB slip transfer mechanisms of multicomponent alloys, which are significantly affected by the GB geometry. In-situ electron backscatter diffraction (EBSD) characterization of the Al-Cu-Li alloys was carried out to reveal the interactions between the dislocations and the GBs. Crystal plasticity simulations and GB precipitation characterization were used to investigate the effect of GB precipitation on slip transfer. The results revealed that GB precipitation significantly changed the slip transfer behavior. The continuous and dense precipitates at some low-energy GBs can alter the interaction mode between dislocations and GBs. Thus, the grain boundary coincident site lattice close-packed plane (GBCP) was proposed to revise the m’(m₁ + m₂) criterion. By introducing the correction coefficient λ as a parameter to modified criterion m’(m₁ + m₂)λ can match the slip transfer phenomenon of grain boundaries under various aging states much better. Combining the experimental and simulation results, a specific GBCP orientation interval is proposed. When the GBCP meets a specific orientation in the inverse pole figure, the GB is likely to transfer slip much better to reach larger elongation and good strengthening effects simultaneously.</p>