Graphitic carbon nitride(g-C3N4) sub-microspheres was first prepared via a facile microwave synthesis through polymerization reaction between cyanuric chloride(C3N3Cl3) and sodium azide (NaN3) using acetonitrile (ACN) as solvent, and the prepared samples were investigated by XRD, FTIR, XPS, SEM, TEM, UV–Vis, PL, TGA and BET, respectively. The results show that g-C3N4 are insoluble to conventional solvents except DMSO, and it exhibits a good chemical stability, thermal stability(< 650 °C), particle size with 0.076–0.137 μm in diameter, surface area of 89.1 m2/g and a band gap of 2.41 eV. Additionally, g-C3N4 prepared by microwave method also displays higher thermal stability, smaller particle radius, larger surface area, lower band gap and stronger emission intensity than traditional solvothermal method. Finally, the effect of microwave on the behavior of C3N4 sub-microsphere is proposed as well.