A casein assay has been developed with aggregation induced emission character of BSPOTPE, which is a derivative of tetraphenylethene (TPE). The method exhibits a wide dynamic range of detection for casein from 10 μg ml(-1) to 5000 μg ml(-1), and a good linear range from 20 μg ml(-1) to 1250 μg ml(-1). With sample-pretreatment procedures, we can detect the casein in skimmed and full milk powder, avoiding the interferences from other components and illegal additives in milk.