Graphitic carbon nitride (g-C3N4) is an attractive photocatalyst, however, its practical photocatalytic applications are still faced with huge challenges. The aim of this research is to identify the correlation between synthetic conditions and properties of the g-C3N4 and derive an optimum synthesis condition for improving photocatalytic activities of the g-C3N4. In this study, novel and versatile g-C3N4 nanosheets were synthesized by the simple thermal pyrolysis of urea. In the synthesis process, the pyrolysis temperature and the heating rate, which can have the most significant influence on the structures and properties of g-C3N4, were set as variables, and the effects were systematically investigated. When synthesized at a relatively high temperature, the amount of material being synthesized is reduced, however it has been found to represent optical properties suitable for highly efficient photocatalyst by the increase in the thickness and defects formed in the g-C3N4 nanosheets. The photocatalytic degradation experiment of MB dyes indicated that the highest degradation of 95.2% after the reaction for 120 min was achieved on the g-C3N4 nanosheets synthesized at 650oC.