Precipitation behaviors and strengthening mechanism in one Al-2.0wt% Li-2.8wt%Cu-0.5wt% Mg and two Al-2.0wt%Li-2.8wt%Cu-Mg-0.5wt%Ag alloys (Mg:0.5 and Mg:1.0) have been investigated by means of Vickers hardness, tensile test and transmission electron microscopy. The microstructure of the alloys primarily consist of $\delta^\prime$, $T_1$ and $S^\prime$ Phase. The coarsening kinetics of $\delta^\prime$ particles well satisfied the time law of the LSW theory and were not greatly influnced by stretching treatment. Addition of Ag and increase of Mg content tends to decrease the size of $\delta^\prime$ particles and their coarsening kinetics. The activation energy for $\delta^\prime$ coarsening in a Al-2.0wt%Li-2.8wt%Cu-0.5wt%Mg alloy is estimated to be $\sim$100kJ/mole regardless of stretch condition. The addition of Ag tends to increase the activation energy for $\delta^\prime$ coarsening to $\sim$115kJ/mole. The activation energy is $\sim$130kJ/mole in an alloy containing Ag and high Mg (1.0wt%). The stretching treatment significantly reduces the volume fraction of $\delta^\prime$ phase. The addition of Ag appears to have no appreciable effect on the $\delta^\prime$ volume fraction. In the case of the alloy containing Ag and high Mg (1.0wt%), high $\delta^\prime$ volume fraction was observed. This is due to the significant reduction of $T_1$ precipitation in this alloy. The stretching treatment greatly accelerates the nucleation kinetics of the $T_1$ phase at the expense of $S^\prime$ phase. The volume fraction of $T_1$ phase tends to increase in the case of Ag containing alloy. These are particularly true in the no-stretch condition. This is probably because of reduction of stacking fault energy by the addition of Ag. Increase of Mg content tends to accelerate the precipitation of $S^\prime$ phase at the expense of the $T_1$ precipitation. The growth rate of the $T_1$phase (and also of the $S^\prime$ phase) is significantly reduced in the stretch condition due to the overlapping of diffusion field. A model has been presented to accomodate this problem. The result is successful to predict the stretch effect on the lengthening of $T_1$ plates. The activation energy for the lengthening of $T_1$ is estimated to be 100$\sim$150 kJ/mole regardless of stretch condition in alloys without and with Ag. A similar value of activation energy (110$\sim$125 kJ/mole) is observed of the growth of $S^\prime$ rods. It is concluded that the growth of both the $T_1$ and $S^\prime$ phases is controlled by Cu diffusion along dislocations regardless of stretch condition. The nucleation of $S^\prime$ phase is greatly influnced by misfit strain energy and it tends to occur at dislocation the Burgers vector of which is nearly parallel to the [010]s $S^\prime$ direction which exhibits the maximum misfit. The variation of yield stress of the alloy has been measured as a function of aging condition. Reverting technique has been employed to distinguish the stretching effect due to the $\delta^\prime$ particles from the overall strengthening effect. The strengthening mechanism of the $\delta^\prime$ particles has been analyzed in terms of the order strengthening mechanism, in which the trailing dislocation pulls completely off from the encountering particles. The addition of Ag and increase of Mg content tends to increase the antiphase boundary energy of $\delta^\prime$ phase.
