This study investigated the effect of weight percent gain (WPG) and compression set on thecrystallity of compressed poplar wood. Tests were conducted to determine the mechanism of compressedwood set. The samples of I-72 Poplar were treated with a PF resin of water-solubility and low molecularweight. Then the air-dried samples were hot-pressed to a certain thickness. Finally the specimens wereobtained from the compressed samples and scanned with an X-ray diffractometer. Results show that beforepress the crystallity of the inner layer is lower than that of the outer layer, and at press the difference of thecrystallity between the outer layer and the inner layer will slowly disappear and finally the crystallity of theinner layer exceeds that of the outer layer; with the increasing compression set the crystallity decreases, andthere is a big decrease; the crystallity difference between two layers happens because of compression setgradient, WPG difference or the joint effect. This study investigated the effect of weight percent gain (WPG) and compression set on the crystallinity of compressed poplar wood. Tests were conducted to determine the mechanism of compressedwood set. The samples of I-72 Poplar were treated with a PF resin of water-solubility then the air-dried samples were hot-pressed to a certain thickness. Finally the specimens wereobtained from the compressed samples and scanned with an X-ray diffractometer. Results show that beforepress the crystallity of the inner layer is lower than that of the outer layer, and at press the difference of the crystallinity between the outer layer and the inner layer will gradually disappear and finally the crystallinity of theinner layer exceeds that of the outer layer; with the increasing compression set the crystallinity decreases, and athere is a big decrease; the crystallinity difference between two layers happens because of compression setgradient, WPG difference or the joint effect.