Abstract: The influence of the molar ratio of formaldehyde to urea on free formaldehyde content (FFC) was studied in strong acid and weak acid. The optimal molar ratio for low FFC was found, and urea formaldehyde resin adhesive with URON structure and low FFC synthesized. The formaldehyde liberation was lower than E1 grade requirement when the adhesive was used in plywood produetion.