The Paleocene-Eocene Thermal Maximum (PETM) event is an abrupt climate change event that occurred at the Paleocene-Eocene boundary. The event led to a sudden reversal in ocean overturning along with an abrupt rise in sea surface salinity (SSSs) and atmospheric humidity. An unusual proliferation of biodiversity and productivity during the PETM is indicative of massive fertility increasing in both oceanic and terrestrial ecosystems. Global warming enabled the dispersal of low-latitude populations into mid-and high-latitude. Biological evolution also exhibited a dramatic pulse of change, including the first appearance of many important groups of “modern” mammals (such as primates, artiodactyls, and perissodactyls) and the mass extinction of benthic foraminifera. Massive input of 12C enriched carbon into the oceanic-atmospheric system induced a prominent negative carbon isotope excursion (CIE) and rapid perturbation of global carbon cycle. The trigger mechanism of PETM remains obscure. One plausible explanation involves abrupt release of marine methane hydrate. Magma activity and bolide effect are also announced accounting for the occurrence of PETM. As for the gas hydrate hypothesis, there are still a lot of different theories explaining how the thermal or depressure dissociation of gas hydrate happened.