Abstract: A mathematical model based on the Lucas-Washburn equation was developed to address the relations of the capillary height,capillary radius and the heat flux in a capillary column and the equation was extended to a capillary loop for investigating the stability of the condensing interface with phase change by some simplifications. The stability of the condensing interface was studied by introducing a small disturbance into capillary height. The dynamics performances of the condensing interface under three different operating conditions were discussed. The results show that there is avibration with a certain amplitude on the condensing interface due to the periodic oscillation of the pressure in the system.