Abstract: Evolvement of flowrate oscillation in parallel narrow rectangular channels was investigated based on the visual experiment with a pair of 3-side visual rectangular channels.The experimental parameter ranges were: system pressure 0.2~0.8 MPa,mass flowrate 60.0~300.0 kg/h,and inlet subcooling 20.0~60.0℃.The evolvement procedure of bubbles and flow pattern transitions in rectangular channel（with a cross section of 40 mm×3 mm） was recorded by the high speed digital camera,while the heat flux of parallel rectangular channels increasing until DWO happened.The experimental results showed that the mass flux of the channels became unsteady as void fraction increasing.Flow transition boundaries,characteristic points of bubble behavior and DWO boundary were associated.