The instability of two-phase loop thermosyphons was investigated experimentally and analytically. Three orifice type inserts were used to study the effect of change in the pressure drop in the flow channel of the TLT on the flow instability and temperature fluctuation. It is observed that a decrease in the size of the orifice insert from 3.7 mm (no insert) to 0.71 mm drastically reduced the fluctuation of the temperature, especially at the evaporator section of the TLT. With the orifice type insert of 0.71 mm for the TLT, the overall temperature fluctuation was almost completely eliminated, especially at higher power input to the TLT The analysis based on the Kelvin-Helmholtz instability theory seems to predict reasonable well the loop stability state of the TLT with experimentally determined constant factors.