Thermal performances of two-phase reciprocating anti-gravity closed thermosyphons of 50% volumetric filling ratio (FR) with/without vacuum are comparatively examined. With different reciprocating frequencies (f) tested; a series of flow snapshots are collected to illustrate the f-dependent temporal variations of flow structures in the reciprocating thermosyphons (RT). At the critical f (f(cr)), the liquid pool in each thermosyphon sways and then starts surging upward to the top-wall under which the confluent hot stream injects downward into the cool liquid pool to facilitate heat exchanges. Acting, together by the additional drags in the shaking liquid due to drifts of immersed air bubbles and the increased air/vapor partial pressures attributing from the heated non-condensable air in evaporator of the RT without vacuum, the f(cr) is raised from the vacuumed RT counterpart. Further f increases to enrich the momentums of liquid streams, the surges of up-lash streams are advanced to counteract/merge with the down splashing stream which bounce off the liquid pool repetitively. The responsive time-mean local and averaged Nusselt numbers (Nu) along the evaporator/condenser centerlines are measured at f = 1.67, 1.83, 1.92 and 2 Hz with sixteen sets of heating/cooling duties at each! tested. Thermal performances in the RT are dominated by reciprocation number (Re-ci) and subject to the interdependent impacts by the heating/cooling duties. With the vacuumed RT, the phase change activities elevate Nu from the non-vacuumed counterparts; whereas the Nu levels are increased by increasing Re-ci, boiling number (Bo) and dimensionless thermal resistance of condenser (R-th,R-con). Thermal resistance properties for present RTs with/without vacuum are examined at various Re-ci, Bo and R-th,R-con with the heat transfer correlations devised to evaluate the averaged Nu over the evaporator section of present vacuumed RT. (C) 2016 Elsevier Ltd. All rights reserved.