Abstract: To confirm the existence of co-condensation in phenol-urea-formaldehyde (PUF) resin and understand the development of molecular structure during the polymerization, the characteristic structure of PUF was analyzed based on the study of structure of phenol-formaldehyde (PF) resin with the help of two main instruments 13C NMR and MALDI-TOF-MS. Analysis results confirmed that the co-condensation among phenol, urea and formaldehyde indeed occurred and formed at the beginning of polymerization. The results also confirmed that structure of copolymer mainly derived from PF resin.