The number and molecular structures of the alpha-brominated 2,5-dimethylterephthalonitrile (DMT) derivatives and the dependences of the yields of derivatives on the reaction time were systematicaly investigated by means of NMR spectroscopy, MS and chromatography. The alpha-brominated DMT derivatives were comprised of 2-bromomethyl-5-methylterephthalonitrile (1), 2,5-bis(bromomethyl)terephthalonitrile (2), 2-(1,1-dibromo)methyl-5-methylterephthalonitrile (3) and 2-bromomethyl-5-(1,1-dibromo)methylterephthalonitrile (4). More than 90 % of DMT mainly transformed swiftly into 1 during the first two-hour alpha-bromination reacting, and nearly half of monobromination compound 1 converts into multibromination compounds 2, 3 and 4 in the ensuing reacting. N-bromosuccinimide (NBS) was not utterly exhausted until the reacting lasted 16 hours. The selection law in the alpha-bromination-reaction of DMT, and the close affinity between the yields of the alpha-bromination products and the reaction time were originally revealed.