A mechanical modelling is proposed in order to describe viscoplastic behaviour without hardening of a nickel-base super alloy loaded at high temperature (900 degrees C) with strain rates varying within a wide range (from 10(-1) to 10(-4) s(-1)). A mathematical law is associated to the viscoplastic model; the parameters of the law are identified from monotonic biaxial tests of membranes loaded by pressure of inert gas (disk pressure testing under helium). The viscoplastic law provides calculated stresses with accuracy better than 1% at the highest strain rates and 4% at the lowest strain rates; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in the bibliography. The parameters identified from biaxial tensile tests of disks have been successfully used to calculate the stresses during stabilized tension-compression loops of cylindrical specimens. The proposed experimental method and behaviour model are interesting because the disk biaxial testing is much more easily performed at high temperature than the tension-compression testing of cylindrical specimens.