As cast FeCrCuMnNi high entropy alloys were heat treated at different temperatures and times. Structure and mechanical properties of the heat treated alloy were studied using X-Ray diffraction, tensile test, hardness test and scanning electron microscopy. It has been found that the FeCrCuMnNi has thermal stability in mechanical properties almost up to 600 degrees C. Nevertheless, the alloy revealed an unusual behavior after heat treating at higher temperatures. A sharp drop of the tensile strength, strain to fracture and work hardening capability occurred at temperatures between 700 degrees C and 800 degrees C due to formation of a hard a phase in the alloy. The hardness follows a plateau in the 700-800 degrees C range and then softening continues, together with a decrease in strength and increase in ductility, due to dissolution of the hard a phase and decreased amount of BCC phase.