This study was carried out to study the effects of different chemical treatments on physical properties and digestion of rice straw. Rice straw was treated with sodium hydroxide (NaOH) at 15, 30, 45, 60 or 75 g/kg dry matter, ammonia bicarbonate (NH₄HCO₃) at 30, 60, 90, 120 or 150 g/kg or their combinations (10 or 20 g NaOH/kg and 30, 60 or 90 g NH₄HCO₃/kg), respectively. The relationship between straw digestion and its physical properties was analysed. Treatment with NaOH increased the volumetric weight (VW), swelling capacity (SC), water-holding capacity (WHC), and in vitro organic matter digestibility (OMD), and decreased specific porosity (SP) of rice straw linearly. Treatment with NH ₄HCO₃ did not significantly affect VW, SC, or SP, but increase OMD linearly and tended decrease WHC. For the combination treatment of NaOH and NH ₄HCO₃, there existed significant interactions on SC, SP and OMD. A significant linear correlation was found between OMD and VW, SC, SP, or WHC, when the data for all treatments were analysed. The accuracy of regression equation could be improved by including multiple physical property parameters. It is inferred that change in all physical properties is indicative of improvement in straw digestion after treatment.