The viscosity values for the experimental cobalt cast alloy Mk1 in the temperature range 1320-1700°C are determined. The effect of heat treatment on the structure of the Mk1 alloy is studied. It was found that during heating at temperatures of 1420 and 1580°C, large carbides M3C2, M7C3 decompose into smaller M23C6. The investigation of the microstructure before and after thermal treatment showed a decrease in the average grain size of the alloy by 2 times.