Zero-dimensional photoluminescent (PL) graphene quantum dots (GQDs) that can be used as the cell-imaging reagent are prepared by a hydrothermal route using the graphene oxide (GO) as the carbon source. Under the optimized hydrothermal conditions, an initial hydrogen peroxide concentration of 0.5 mg mL(-1) at 180 degrees C for 120 min, the GO sheets can be cut into nanocrystals with lateral dimensions in the range of 1.5-5.5 nm and an average thickness of around 1.1 nm. The as-prepared GQDs exhibit an abundance of hydrophilic hydroxy and carboxyl groups and emit bright blue luminescence with up-conversion properties in a water solution at neutral pH. Most interestingly, they indicate excitation-independent emission characteristics, and the surface state is demonstrated to have a key role in the PL properties. The fluorescence quantum yield of the GQDs is tested to be around 6.99% using quinine sulfate as a standard. In addition, the as-prepared GQDs can enter into HeLa cells easily as a fluorescent imaging reagent without any further functionalization, indicating they are aqueous stability, biocompatibility, and promising for potential applications in biolabeling and solution state optoelectronics.