Zero-valent iron nanoparticles ( nanoparticles) with particle sizes in range 2–5 nm were synthesized using guava leaf extract from the reduction of soluble precursor. The most effective solvent for the extraction of polyphenolic compounds from guava leaf was identified to contain 70% hydroethanolic solution. It was also found that this extract gives high reduction potential and a strong chelating ability for the preparation of nanoparticles from soluble in basic aqueous solution at room temperature. At pH 8, the deprotonation of the polyphenolic groups with terminal –OH provides the strong binding sites to form a stable complex with and can take part in the redox reduction. The formation and growth of nanoparticles were monitored by UV-visible absorption spectroscopy and transmission electron microscopy. The achieved high dispersion and narrow size distribution of nanoparticles are attributed to the ability of tannins which are the major components of the polyphenolic groups to form strong complex with and stabilize nanoparticles from aggregation. Methylene blue degradation in an aqueous solution was used as a model for study catalytic reduction performance of the synthesized tannin-stabilized nanoparticles in colloidal form.