A rapid synthesis process of copper nanoparticles (CuNPs) using green agents was investigated in the present work. In the synthesis process of CuNPs, L-ascorbic acid and starch were used as reducing and stabilizing agents, respectively, while the presence of microwave irradiation enhanced the formation rate of CuNPs. A starch concentration of 10 g/L, 20 mM Cu(AcO)₂, 80 mM L-ascorbic acid, the power level of 360 W, and reaction time of 120 s were found as optimal conditions for CuNPs formation. Under the optimum condition, the synthesis efficiency of CuNPs was approximately 89.7%. The synthesized CuNPs possessed an absorption peak at a maximum wavelength of 602 nm and spherical shapes with an average size of 13.6 nm. The quasi-sphere CuNPs were produced with a pure phase. Additionally, the synthesized CuNPs heightened antifungal activity on plants including Colletotrichum gloeosporioides (C. gloeosporioides) and Fusarium solani (F. solani). The antifungal activity of CuNPs against F. solani was better than that of C. gloeosporioides. The best inhibitory concentration for F. solani was 1.0 mM while the value of 1.5 mM CuNPs was found to be the best value for C. gloeosporioides inhibition. The obtained results indicated that the CuNPs are effective nanoparticles against fungal infection.