<p>Reduced flowability or caking of grain by-products, such as distillers’ dried grains with solubles (DDGS), wheat bran (WB) and wheat germ (WG), causes many problems in their manufacturing process. Physical characteristics of grain by-products were determined and effects of glass transition temperature (T_g) and water activity (a_w) on the flowability and caking were investigated. All three samples showed typical type II curves in the moisture sorption isotherm (MSI). DDGS showed strong hygroscopicity at relatively high a_w, compared to WB and WG. In addition, as the temperature increased, all three samples became more hygroscopic at high a_w. When T_g was determined with varying a_w, all samples revealed the low sensitivity to the plasticization by water. Tap density test and tilting angle test were performed to determine the flowability, and the tilting angle test was found to be the adequate method in this case. Environmental and intrinsic factors did not significantly affect the flowability of sample and the critical point of the flowability was thought to be closely related to the hygroscopicity of samples. Unlike flowability, caking characteristic was influenced by temperature with changing a_w, suggesting that the rubbery state is a major factor in caking of the grain by-product powders. Consequently, controlling moisture condition is the most important factor to maintain flowability and to prevent caking of grain by-product powders.</p>