The effect of ’ particle size upon the mechanical properties of Ni base superalloy EP741NP obtained by powder metallurgy was investigated. The particle size of ’ phase in -’ microstructure was varied by changing the cooling rate V from the temperature of the solid solution treatment at 1200 °C (V = 80, 200 and 400 °C \ min.). After solid solution treatment billets were subjected to aging in the standard mode. It was established that as V increases from 80 to 200 °C \ min., the average particle’s size of ’ phase decreases from 0.54 microns to 0.22 microns in the aged state. This improves the characteristics of creep and low cycle fatigue at 650°C: time to rupture under load 1000 MPa increased from 132 hours to 416 hours and low cycle fatigue increased from 42,215 to 82,016 cycles.