[en] When a natural circulation system is operated under low pressure and low steam quality, the oscillation behavior of the flow rate of the steam-water mixture in this system is studied. The experimental work was done using water as coolant on the test system HRTL-200 under 1.0-4.0 MPa of pressure, 27-190 kw of heating power, 5-80 degree C of inlet subcooling and less than 5% of outlet steam quality. The range of experimental parameters covers those of the district heating reactor 200 MW. The experimental results about the effect of system pressure, heating power and coolant subcooling on the relevant amplitude of natural circulation flow rate and the oscillation periods are presented