This experimental study comparatively examines the thermal performances of two-phase loop thermosyphons (TPLP) with and without enhanced boiling surface at sub-atmospheric pressures. The boiling instabilities along with the constituent and total thermal resistances of these TPLPs are analyzed with the aid of boiling flow structures imaged at sub-atmospheric pressures. Boiling heat flux (Q) and thermal resistance of condenser (R
) are selected as the controlling parameters with their individual and interdependent effects on the thermal performances examined. With the present enhanced boiling surface, the intermittent bursting of large bubbles from liquid pool in the multi-channel evaporator of plain surface is significantly suppressed, leading to the moderate pressure waves agitated by bubble eruptions with reduced boiling instabilities and pressure-drop thermal resistances (R
). The effects of TPLP height (H), which affects the driven pressure head for liquid-vapor circulation, on the thermal performances of the enhanced TPLP at various Q and R
are subsequently examined. Total thermal resistances (R
) measured from the TPLTs with enhanced boiling surface are considerably reduced from the TPLTs with plain boiling surface and reduced to about 0.265 at the test condition of Q=150W, R
=0.2, H=35.3 tube diameters. A set of R
correlation which permits the evaluation of individual and interdependent Q, R
and H impacts on total thermal resistances of the enhanced TPLPs is generated to assist the design activities using this type of enhanced TPLP for cooling of electronic chipsets.