The effects of cellulose powder and water activity (aw) on the equilibrium water content, glass transition temperature (Tg), mechanical relaxation, and caking of freeze-dried amorphous carbohydrate blend powders (dextrin-glucose mixture) were investigated. Water sorption isotherm and Tg-curve were obtained, and critical water activity (awc) was determined as the aw at Tg = 25 °C. There was a minor effect of cellulose powder on the awc. The degree of caking was negligible below the awc (glassy state), but increased remarkably above the awc (rubbery state). Cellulose powder played a dispersive role in the powder, and thus diminished the degree of caking. The caking behavior could be described by a stretched exponential function, and an experimental formula to predict the degree of caking as a function of aw/awc was obtained. The results for a food powder, with a more complex composition, largely deviated from the formula. Mechanical relaxation was evaluated as a force-reduction during isothermal compression. It was found that the relationship between the degree of caking and force-reduction could be described as a linear function independent of aw and cellulose powder content. The results for the food powder partially followed the linear relationship.