The two-phase closed loop thermosyphon is investigated with emphasis on the overall performance in transient operation. The control volume approach is the base of a global analysis describing the motion of vapor and liquid phases of the thermosyphon system in one-dimensional equations. Interfacial shear forces are neglected as only co-current flows are present. Heat transfer coefficients are based on empirical correlations. It is found that the density ratio vapor-liquid, dimensionless friction coefficient and water column length determine respectively the overall dynamic behavior characteristics such as response time, damping and oscillation frequency.