Thermally stimulated luminescence (TSL) of BeO powders and ceramic samples of density 2.10 - 2.93 g/cm3 made of the same batch of powder is investigated. It is established that with increasing density of BeO samples (and, accordingly, increasing mean size of microcrystals), as well as with increasing vacuum in a cryostat chamber, the positions, number, and intensity of low- and high-temperature TSL peaks of adsorption and non-adsorption origin are modified. It is assumed that impurity and adsorption centers of volume and surface origin, as well as internal lattice defects, are responsible for the formation of TSL centers in BeO powders.