PURPOSE: To obtain a finely particulate brominated polystyrene which can be mixed into a resin having a low molding temp. without detriment to the physical properties of the resin by pulverizing a brominated polystyrene obtd. by the polymn. of a brominated styrene monomer.

CONSTITUTION: This finely particulate brominated polystyrene having an average particle size of 5μm or lower and a wt. average mol.wt. of pref. 10,000-200,000 is produced by pulverizing a brominated polystyrene obtd. by the polymn. of a brominated styrene monomer represented by the formula (wherein (x) is 1-5). A brominated polystyrene usually has a relatively large particle size, 0.5mm to several mm, and when used as a flame retardant for a resin which is molded at a temp. lower than the softening point of the brominated polystyrene, it can not be mixed into the resin, or even if it is miscible, the physical properties of the resin is markedly degraded. The brominated polystyrene with a particle size of 5μm or lower can be mixed into a resin having a low molding temp. without degrading the physical properties of the resin.