To improve our understanding of natural gas hydrates dissociation characteristics under vacuum pressure, preliminary experimental studies on the decomposition of synthetic hydrates including methane hydrates, ethane hydrates, propane hydrates, normal butanenitrogen hydrates, iso-butane hydrates, mixed gas hydrates, methane hydrates in various size grained porous media, and natural hydrates recovered from Shenhu sea area in South China Sea and Qilian Mountain permafrost, were conducted in a vacuum device placed in a constant temperature room. The experimental results reveal that except propane hydrates and iso-butane hydrates, the decomposition processes of synthetic methane hydrates, ethane hydrates, normal butanenitrogen hydrates and mixed gas hydrates are similar, and the pressures of the decomposing gases appear monotonically increasing at vacuum condition; however, during the dissociation processes of synthetic methane gas hydrates in porous media, the increasing rate of the decomposing gases pressures generally show “fast-slow-fast” feature and the peculiarity of gas hydrates self-preservation are obvious; similarly, natural hydrates recovered from Shenhu sea area in South China Sea and Qilian Mountain permafrost also reveal apparent selfpreservation during the dissociation processes，but different from natural hydrates Qilian Mountain permafrost, the decomposing gases pressures of natural hydrates recovered from Shenhu sea area increase in a ladder form. The differences between both natural hydrates may be due to different distribution models and occurrences of hydrates in the cores.
