The tensile properties of a forged UDIMET 720Li alloy have been investigated at room temperature. The aim of this study was to increase both yield and ultimate tensile stresses using adequate thermal treatments. Classical three steps heat treatments after hot forging were applied: a solution treatment followed by a quench and a two steps aging treatment. Several combinations were investigated: four hours solution treatments, either sub-solvus (1080 C -1120 C) or super-solvus (1160 C); two different cooling rates (10 C/min or 3600 C/min); four different two-steps aging treatments: 650 C/24h/Air Quench (AQ) + 760 C/16h/AQ, 760 C/16h/AQ + 650 C/24h/AQ, 700 C/24h/AQ + 815 C/16h/AQ, 815 C/16h/AQ + 700 C/24h/AQ. The tensile properties appeared to be maximized with the following combination of heat treatments: sub-solvus solutioning (1120 C or 1080 C), fast cooling rate (oil quench, 3600 C/min), and the 760 C/16h/AQ + 650 C/24h/AQ aging. Both EBSD measurements and systematic stereological analyses were performed to characterize for each condition, grain size and γ' distribution (primary γ', secondary γ', tertiary γ'), respectively. The increase of the 0.2% yield stress, is managed by a competition between keeping a small grain size (i.e. by using a low temperature sub-solvus solution treatment) and increasing the intragranular γ' content (i.e. by increasing the solutioning temperature). In order to evaluate the relative contributions to the deformation mechanisms of grain size and of γ' particles, especially intergranular one (primary γ'), Scanning Electron Microscope (SEM) insitu tensile tests have been performed at room temperature on both sub-and super-solvus samples. It is finally demonstrated that the main controlling parameter to reach a very high yield stress is grain size.