The effects of $\gamma$' heat treatment on the microstructure and the mechanical properties of a newly developed mechanically alloyed(MA) oxide dispersion strengthened (ODS) Ni-base superalloy were investigated. The composition of MA ODS Ni-base superalloy investigated in this study was Ni-5Co-8Cr-5.5Al-1Ti-6Ta-1Mo-8W-2Re-0.03B-0.01Zr-0.9$Y_2O_3$ and the alloy was received as hot extruded bars after mechanical alloying from Inco Alloys International, Inc. in USA. And zone annealing was used with the maximum temperature of 1300℃ and the zone traveling speed of 65mm/hr to develop elongated coarse grain structure. In order to improve the intermediate temperature mechanical properties of Ni-base superalloys, it is important to control the size and the distribution of $\gamma$' precipitates by heat treatment. The heat treatments of Ni-base superalloy are consisted with solution treatment and aging treatment. In this study the temperature and the cooling rate of solution treatment and the temperature and the time of aging treatment were changed. The primary $\gamma$' precipitates formed during zone annealing were dissolved and new $\gamma$' precipitates were formed during the cooling stage from solution treatment temperature. And the coarsening of $\gamma$' precipitates proceeded in age treatment. When the solution treatment temperature was above 1260℃, the primary $\gamma$' precipitates were completely dissolved and the size distribution of $\gamma$' precipitates after solution treatment became uniform. When the solution treatment temperature was below 1260℃, a bimodal size distribution of $\gamma$' precipitates consisted with large undissolved primary $\gamma$' precipitates and small newly nucleated $\gamma$' precipitates was observed. The volume fraction of undissolved primary $\gamma$' precipitates could be controlled by the change of the temperature. By 954℃ aging treatment the size of $\gamma$' precipitates could be controlled by coarsening but the coarsening of $\gamma$' precipitates was decelerated because of the effect of elastic interaction energy from lattice misfit between $\gamma$' precipitates and r matrix. The yield stress in compression and tension test with $\gamma$' precipitates size was analyzed. $\gamma$' precipitates less than 0.25μm was sheared by paired dislocation and $\gamma$' precipitate larger than 0.25μm was deformed by Orowan by-pass mechanism. And the yield stress of bimodal size distribution $\gamma$' precipitates could be explained by superposition of strengthening and the rule of mixture.
