This work presents the theoretical and experimental results of research upon a flow of the mixture of water and steam water. The experiments were performed in the model of an evaporator and in the real evaporator of a once-through steam boiler BP-1150. The total pressure drop was investigated in a vertical tube of the steam boiler to which a constant heat flux was fed. The measurement results and the computer simulations showed that the two-phase flow instability occurred both in the evaporator model and in the real steam boiler evaporator.