The improvement of thermal management action during the accident of nuclear power plants (NPPs) becomes important consideration including improving the inherent safety on NPPs for operation, especially in cooling process. The use of natural circulation as an alternative for thermal management and operating of NPPs, especially in SMRs becomes an interesting research and development topic of safety engineering. The leak of information related the characteristic of flow stability in natural circulation for use in passive safety system must be solved for implement this phenomenon in the real applications. Experimental method was used to determine the characteristic of natural circulation flow rate due to the temperature differences at two points. To clarify and understand the phenomenon, FASSIP-01 loop was constructed; with two parameters such as thermal-hydraulics represent by temperature and geometrical effect such as total pipe length, pipe diameter and height differences of cooler and heater. The purpose of the experiment is to understand the effect of power variation related into time respond from start-up until stable flow. The variations of heater power in this experiment are 3840 watt, 4240 watt, and 5928 watt with optimal cooling flow in secondary side (HSS flow) is 43.10 LPM and HSS flow variation from 21.57 LPM, 33.63 LPM and 43.10 LPM at heater power 5928 watt. The result of experiment shows It is show that, the effect only related to respond time for stability flow from instability flow. The increasing HSS flow in cooler area to respond time from 7325 s to 5041 s is more fast only 31.18%. In case of heater power increasing in heater area to respond time from 4855 s to 3857 s is more fast only 20.55%.