A castor assembly includes a castor unit, a positioning unit, and a brake unit. The castor unit includes a housing and two rollers. The housing has a neck portion and a body portion. The positioning unit includes a fixing block located under the neck portion, a toothed plate pivotally connected with the body portion, and two restoring springs biased between the toothed plate and the body portion. The fixing block has an annular toothed portion. The toothed plate has a plurality of locking teeth that are movable to engage the toothed portion. The brake unit includes a braking block having two braking faces press the rollers. Thus, the toothed plate presses the fixing block by the elastic force of the restoring springs so that the locking teeth of the toothed plate engage the toothed portion of the fixing block closely.