Flooding may take place for in-tube condensation, causing unstable flow and deteriorated heat transfer. Here, the phase separation principle is proposed to eliminate flooding. Comparative experiments of condensation were performed in both bare tube (BT) and modulated heat transfer tube (MHTT) with mesh membrane tube insert (MMT). The working fluid is water-steam under sub-atmospheric pressure. It was observed that slug flow exists at small mass fluxes and vapor mass qualities in BT. Due to the periodic formation of liquid column over the tube cross section, flooding indeed takes place, causing unstable flow, deteriorated heat transfer and large pressure drop. The MHTT completely eliminated flooding, converting unstable flow into stable flow. Heat transfer coefficients are 7.47 times of those in BT, maximally, accompanying reduced pressure drops. High-speed visualization and theoretical analysis indicated that smaller pore size provides larger capability to prevent vapor phase penetrating mesh screen, resulting in larger driving force for liquid suction towards the MMT inside. The MMT provided a tunnel for liquid transportation in upward direction, which is the mechanism to eliminate flooding. The modulation of annular flow pattern was also performed and analyzed by using the phase separation principle, and the results indicated the effectiveness of MMT in annular flow regime. This work is benefit to applications such as air-cooled condenser, whose performance is important to influence the whole system performance for power generation.