In hot molding processes of woody material, it is important to understand the effect of oven-dry heating on the property of woody biomass material, such as thermal fluidity. In this study, thermal flow tests of untreated and steam treated bamboo powder were conducted to investigate the effects of heating at an oven-dry state on thermal fluidity. The test temperature was set to 200°C. Before the thermal flow test, powder was dry-heated in a capillary rheometer at 200°C with a variable heating time. Thermogravimetry was conducted to understand the thermal changes of the powder during an increasing temperature and constant temperature. Fluidity of untreated powder was improved with a short dry-heating but decreased with a long dry-heating. In contrast, steam treated powder fluidity was high compared to untreated one, but its fluidity did not improve from dry-heating. From these thermogravimetry results, the chemical changes associated with component volatilization relate with the thermal fluidity. Therefore, the decrease in fluidity from dry-heating occurred because fluidity related components escape from the powder through volatilization.