This study compares copper nanoparticles (CuNPs) synthesized by green and mechanochemical methods. CuNPs were produced using Hibiscus sabdariffa and Capsicum annuum extracts and by high-energy ball milling. X-ray diffraction confirmed face-centered cubic copper, with minor cuprous oxide (Cu2O) in milled samples. Green synthesis in 10 min yielded smaller, spherical CuNPs (51–60 ± 1 nm) stabilized by phytochemicals, while 75 h of milling produced larger, irregular particles (~85 ± 2 nm) with partial oxidation and aggregation. Structural and compositional features were analyzed by multiple techniques. Results show how synthesis method shapes particle size, morphology, and oxidation.