Precipitation strengthening mechanisms using tensile test and transmission electron microscopy (TEM) were investigated in two alloys of Al-2.33Li-2.48Cu(-0.09Zr) and Al-2.39Li-1.50Cu(-0.09Zr). In both cases, primary strengthening originates from two different types of precipitates of δ` and T₁ phases. Retrogression technique has been employed in order to estimate the contribution of each types of precipitates. The strength of the alloys increased by applying a small amount of tensile stretch after the solution treatment prior to aging. This was shown to be primarily due to the enhancement of the nucleation rate of the T₁ phase. Results suggest that the chemical strengthening is the principle strengthening mechanism of the T, precipitates. Calculated interfacial energy of the edge of the T₁ phase was 2.82 J/㎡. Strengthening by δ` precipitates was best explained by the mechanism of the order strengthening in which the trailing dislocations pull off immediately from the encountering particles. The addition rule of the contributions from these distinctively different obstacles has been tested and was found to be best satisfied with the addition exponent, q=1. 4.