The effect of evaporator tilt on the operating temperature of a loop heat pipe (LHP) without a secondary wick under terrestrial surroundings was investigated both experimentally and theoretically in this work. The experiments were conducted with the evaporator placed at three different tilts: (a) the evaporator was horizontal with the compensation chamber (CC), (b) the evaporator was vertically below the CC, and (c) the evaporator was a little higher than the CC with a tilt angle of 1.80 degrees. Experimental results show that the evaporator tilt has significant effect on the operating temperature of the LHP: the operating temperature at evaporator tilt (c) was much higher than those at two other evaporator tilts. Theoretical analysis based on a two-zoned evaporator wick, i.e. a subcooled zone and a saturated zone, is conducted to explain the experimental results, indicating that the cooling effect of the returning liquid on the vapor region in the CC or the evaporator core is crucial in determining the operating temperature of LHPs, which should be well considered in the LHP design and applications. (C) 2014 Elsevier Ltd. All rights reserved.