Abstract: The test section consisted of two heated rectangular channels connected in parallel and paralleled by an unheated by-pass.Density wave oscillation in parallel rectangular channels was systematically observed on a two phase flow test rig operated at system pressures ranging from 1 to 10 Mpa.The oscillatory behavior is analyzed as well as the effects brought on the threshold of instability by changes in the operating system pressure,inlet subcooling number and mass flow.Moreover,this experimental data was compared with that inform the two parallel rectangular channels by using dimensionless subcooling number Nsub and phase change number Npch.The comparison indicates that the existence of by-pass makes the system more stable.