Abstract: An accurate, convenient and fast method was proposed for the determination of tetracyclines (tetracycline, chlortetracycline, oxytetracycline, doxycycline and methacycline) and their preparations by flow injection detector based on tetracycline flow- through sensor. The parameters affecting the measurement were discussed. The detector can repond to tetracycline, chlortetracycline, oxytetracycline, doxycycline and methacycline, at the same time. Their slopes are 51～ 55 mV/decade over the concentration range of 10^-2～5×10^-5mol/L at pH 1.5～3.5. The results obtained are in good agreement with those by biological assay (less than 3% deviation ). One hundred samples can be determined in an hour.