Abstract: Through study on C-13 NMR and chemical analysis of Urea-Forma-ldehyde Concentrates (UFC), it is clearly shown that (1) UFC is a solution of formaldehyde, additive compounds of urea and formaldehyde, and prepolymer with low degree of polymerization and linkage of dimethylene ether (—CH₂OCH₂—), in which tri- and disubstituted urea are in the majority and tetra- is in the minority; (2) C-13 NMR of UFC supplies a quantitative determination to agree with that of chemical analysis, and more information about the constitution of UFC than it.