The structure of cashew nut shell liquid in the modified phenol-formaldehyde resin has been investigated by infrared spectroscopy. The Mark-Houwink expressions for the modified phenolformaldehyde resin in acetone at 250℃ were found to be [η] = 6.93 × 10-2 Mn0.59 for fractionated samples with a molecular weight range of 710-2740 by viscometry and vapor pressure osmometry.The parameter a of Mark-Houwink equation and Huggins constant Kn were discussed.