Copper nanoparticles (CuNPs) possess a promising antibacterial and antifungal activity with low-cost reagent and fabrication. In this study, we report the antifungal activity of pure CuNPs against Fusarium oxysporum (F. oxysporum) and Phytophthora capsici (P. capsici). CuNPs were prepared by reducing Cu²⁺ from copper(II) chloride dihydrate via a green chemical reduction agent using ascorbic acid as both reducing agent and antioxidant agent in aqueous media and polyvinylpyrrolidone as a stabilizer. The effect of the solution pH, reduction time, ascorbic-acid-to-Cu²⁺ molar ratio, role of stabilizing agent polyvinylpyrrolidone to particle size of CuNPs were studied. The antifungal activity of CuNPs at different concentrations and different particle sizes against two plant pathogenic fungi F. oxysporum and P. capsici has been tested by the agar disc diffusion technique. Characterizations were carried out by X-ray diffraction measurements, transmission electron microscopy, and ultraviolet–visible light, which revealed that the CuNPs obtained at different synthesis conditions have different particles size, resulting in different antifungal activities. The synthesized CuNPs demonstrated significant antifungal activity against F. oxysporum and P. capsici. F. oxysporum and P. capsici were entirely inhibited at the concentration of CuNPs of 30 ppm after 3 days of incubation and 7.5 ppm after 1 day of incubation, respectively. Our results suggest that synthesized CuNPs can be used as a novel antifungal agent in agriculture to control the plant pathogenic fungi of F. oxysporum and P. capsici.