This paper presents a theory that explains the low frequency, high amplitude temperature oscillations in loop heat pipe (LHP) operation. Temperature oscillations with amplitudes on the order of tens of Kelvin and periods on the order of hours have been observed in some LHPs during ambient testing. There is presently no satisfactory explanation for such a phenomenon in the literature. It is well-known that the operating temperature of an LHP with a single evaporator is governed by the compensation chamber (CC) temperature, which in turn is a function of the evaporator power, condenser sink temperature, and ambient temperature. As the operating condition changes, the CC temperature will change during the transient but eventually reach a new steady state. Under certain conditions, however, the CC temperature never reaches a true steady state, but instead displays an oscillatory behavior. The proposed theory explains why low frequency, high amplitude temperature oscillations may occur when the LHP has a large thermal mass attached to the evaporator, a low applied heat load, and a very cold sink. When this condition prevails, there are some complex interactions among the CC, condenser, thermal mass and ambient, which allows the thermal mass to modulate the constant applied heat load into an oscillatory heat input to the evaporator. The oscillatory evaporator power is the source of the temperature oscillation, which can be correlated to the vapor front movement in the condenser. The theory agrees very well with previously published test data. Effects of various parameters on the amplitude and frequency of the temperature oscillation are also discussed.