We show the control of a charging energy in chemically assembled nanoparticle single-electron transistors (SETs) by altering the core diameter of Au nanoparticles. The charging energy is a fundamental parameter that decides the operating temperature of SETs. Practical application of SETs requires us to regulate the value of the charging energy by tuning the diameter of quantum dots. In this study, we used 3.0, 5.0 and 6.2 nm diameter synthesized Au nanoparticles as a quantum dot in the SETs. The total capacitances and charging energy of the SETs were evaluated from the rhombic Coulomb diamonds attributed to a single Coulomb island. The capacitance and charging energy matched with a concentric sphere model much better than with a simple sphere model. The operating temperatures of the SETs suggested that a charging energy 2.2 times greater than the thermal energy was required for stable operation, in theory. These results will help us to select an appropriate core diameter for the Au nanoparticles in practical SETs.