Abstract: In weak acid medium,tetra-carboxylic copper phthalocyanine reacts with gentamycin sulfate form an ion association complex and its resonance light scattering (RLS) intensity enhances greatly. Based on this phenomenon, a novel method for the determination of gentamycin sulfate was developed.The RLS signal reached to the highest intensity at the wavelength of 362 nm, and the intensity of which was proportional to the gentamycin sulfate concentration in the range of 0.05 to 1.50 μg/mL，with a detection limit of 0.044 μg/mL. The proposed method has been applied to the determination of gentamycin sulfate injections with satisfactory results.