High pressure Raman study on graphene nanoplates, graphite and micro-graphite has been carried out in a diamond anvil cell. A phase transformation has been observed in graphene nanoplates at 15 GPa in the experiments with or without pressure medium, which can be explained by the interlayer coupling with sp(3) bonds formed in the material. For graphite and micro-graphite, the transition pressure is 19 GPa. Different transition pressures for them are attributed to the thickness difference. The lower transition pressure in graphene nanoplates has been discussed in the framework of the special limited-number layer structure and nucleation process in phase transition. (C) 2013 Elsevier B.V. All rights reserved.