The strength, or the effective height,of pulse boiling in thermosyphon was studied theoretically and experimentally based on the previous studies about the pulse boiling phenomenon. A physical model was presented, in which the forces of gravity, friction and pressure difference for a control volume were considered approximately. The one dimensional control equation was derived from a series of assumptions and the resultant expression of pulse boiling strength was obtained. It was found that the effective height of pulse boiling is affected by heat flux, operating pressure, filling rate and diameter of tube. In order to avoid the effect of pulse boiling on the heat transfer of condenser zone, a critical heat flux, or a limitary heat flux, was presented.