The intact and steam-flaked sorghum grains were analysed for starch crystal structure, gas production kinetics and ruminal fermentation characteristics to investigate the disintegration of starch crystal structure by steam flaking responsible for the improved in vitro ruminal fermentation of steam flaked sorghum. All typical diffraction peaks at 2θ values of 15°, 17°, 18°, and 23° disappeared in the steam-flaked sorghum. The degree of starch gelatinization and the rate of gas production were significantly increased (P<0.0001) by steam-flaking processing. The steam-flaked sorghum showed increased ruminal molar proportion of acetic acid (P<0.01) and propionic acid (P<0.01), and decreased acetic: propionic acid ratio (P<0.05). The present results indicate that the disintegration of starch crystal structure, associated with increased starch gelatinization in steam-flaked sorghum, would be responsible for the improvement of in vitro ruminal fermentation of steam flaked sorghum grains.