Solvothermal reduction of graphene oxide (GO) is a convenient green method to produce graphene. It has been found that the autogenous pressure in the sealed vessel can accelerate reduction of GO comparing to the same reaction in an open vessel at the same temperature. However, the underlying mechanism is unknown. We have investigated the solvothermal reduction of GO in aqueous dispersion in the absence of reductants in both open and sealed vessels at the same temperature. We found that the solvothermal reduction of GO made it easy to sonicate the reduced GO sheets into small pieces. Atomic force microscopy (AFM) and dynamic light scattering (DLS) results show that the high pressure not only accelerates the reduction of GO, but also makes the reduced GO sheets easily become broken by sonication into smaller pieces with a narrower size distribution. Infrared (IR) spectra shows no apparent difference of CO stretching and O—H bending between the reduced GO under both conditions. However the C—O—C stretching of GO was significantly decreased after reduction at high pressure. We propose that C—O—C bonds of GO can be converted to C—OH, which is an addition reaction with water molecules, therefore can be accelerated at high pressure. The epoxyconverted C—OH is then removed in the form of CO2, leaving lines of defects on the GO sheets and making the GO sheets more easily be broken into small pieces at those sites.