Effects of thermal pre-treatment on physicochemical properties of okara insoluble dietary fiber (IDF) during wet media milling were studied. During the milling process, control sample (without pre-treatment) showed the largest mean particle size, followed by samples under pre-treatment temperature at 80 degrees C, 100 degrees C and 120 degrees C, respectively. The minimum size (298.17 inn) was obtained under the temperature of 120 degrees C and milling time of 6 h. The IDF samples subjected to thermal pre-treatment showed disintegrated and irregular surfaces, and were presented in agglomerated form with puffed morphology after subsequent milling for 6 h. Fourier transform infrared spectroscopy and X-ray diffraction analysis revealed the modification of structural characteristics and the degradation of IDF components after thermal pre-treatment and wet milling. As the milling duration prolonged, water soluble index, Zeta potential and apparent viscosity of all the samples decreased, while swelling power abruptly increased and then gradually dropped.