The period operation of power electronic acts as switching element, where the power dissipated consists of pulses at certain duty cycle, the semiconductor temperature oscillates and varies as a waveform. In the present study, an experimental investigation was carried out for a loop thermosyphon order to evaluate the effect of pulsate surface heat flux on the single phase buoyancy driven convection of ethylene glycol flow through a minichannels heat sink with hydraulic diameter 1.5 mm. An electric heater block is used to supply the heat flux to minichannels heat sink in a rectangle waveform. The study is done at different heat flux frequencies of 2.777×10 3 Hz, 8.333×10 4 Hz, 5.555×10 4 Hz and 4.166×10 4 Hz, while the heat flux amplitude (2 watt), Rayleigh number (1864) and duty cycle (50 %) are kept constant. The results revealed that for a range of the measured frequency for the complete power cycle and due to unsteady state operation conditions, the pulse heat flux pattern is close to a rectangle wave, this generates the fluid outlet temperature pattern close to a triangle wave. The fluid outlet temperature increases with the decreases of heat flux frequency and tends to reach to the fluid outlet temperature for a constant and continuous heat flux case. Due to closed loop of thermosyphon, the fluid inlet temperature is changed in pattern like that the fluid outlet temperature change.