The geyser boiling instability in a two-phase closed loop thermosyphon (TPCLT) is experimentally investigated through flow visualization and simultaneous measurement of pressure and temperature fluctuations. Wide ranges of filling ratios of R134a fluid from 90% to 53%, and heat flux from 20 W cm(-2) to 220 W cm(-2) are examined. The Power Spectrum Density (PSD) method is applied to analyze the periodicity of geyser boiling, and the parameter Standard Deviation (SD) is used to characterize the oscillating amplitude. The effects of heat flux and filling ratio on the geyser boiling occurrence and the oscillation characteristics are discussed in detail. The results show that, the flow regimes experience the consecutive variation of single-phase flow, bubbly flow, churn flow, bubbly flow, and single-phase flow within each geyser boiling cycle, leading to the fluctuation of flow and heat transfer characteristics. The geyser boiling is more liable to occur in the conditions of higher filling ratio and moderate heat flux. The initiating heat flux for the onset of geyser boiling decreases with the increase of filling ratio, but the range of heat flux for the geyser boiling occurrence is narrow under the high filling ratio conditions. The frequency of geyser boiling increases with the increase of heat flux for a certain filling ratio. With the increase of filling ratio, the oscillating frequency firstly decreases and then increases. The minimum oscillating frequency occurs at the combination of filling ratio of 76% and heat flux of 90 W cm(-2) under the experimental conditions in this work. Both the fluctuation amplitude of pressure and temperature increase with the increase of heat flux, while decrease with the increase of filling ratio. Compared to R134a under the same filling ratio of 76%, the water has higher heat flux for geyser boiling occurrence, smaller oscillating amplitude of pressure, and lower oscillating frequency due to the difference in thermophysical properties. (C) 2018 Elsevier Ltd. All rights reserved.