Abstract: By means of Fourier transform infrared(FTIR) spectroscopy, the structure development of urea-formaldehyde (UF) resins in synthesis process was investigated, and the structural difference of reaction inter-products of urea and formaldehyde was observed in alkaline and acidic media and at different formaldehyde-to-urea molar ratios. With the reaction process of urea and formaldehyde the variation of the FTIR peaks was obvious, the amide II band frequency of FTIR spectra of UF resins shifted gradually from high to low, and more methylols, ether linkages and —NHCH2— were formed. But in the cured UF resins, the contents of methylols, ether linkages, and —NHCH2— decreased. With the increase of the formaldehyde-to-urea molar ratios from 0.5 to 3.0, the amide II band frequency also shifted gradually from high to low, and the contents of ether linkages and —NHCH2— increased in alkaline media, while in stronger acidic media the contents of Uron, methylene linkages, and methylols increased, but the content of —NHCH2— decreased.