The method of inspecting and evaluating an oil storage tank currently focuses on the bottom and outer wall of the tank， and the roof the tank does not typically receive sufficient attention. However， the complicated structure and the unique physical and chemical environment lead to corrosion of the roof， which in severe cases may result in sudden failure or fire. Estimating the extent of corrosion of a tank roof is therefore of great significance. A new method based on the Lamb-wave reflection/transmission amplitude ratio is proposed to evaluate the corrosion depth. By analyzing the nonlinear modulation of guided waves in corroded materials， the relationship between corrosion depth and the reflected and transmitted wave amplitudes is obtained. Then， using the attenuation characteristics of guided waves， a theoretical expression for the ratio of reflected and transmitted signal amplitudes is derived. Numerical analysis and electrochemical corrosion experiments verified the relationship between the corrosion depth and the amplitude ratio. Using realistic experimental parameters and assuming a plate thickness of 3 mm， it is noted that guided wave attenuation factors （β） equal to 0.5， 1 and 2 are capable of distinguishing the degree （i.e.， depth） of corrosion. A practical test was conducted at a petrochemical facility； an ultrasonic guided wave categorization of corrosion into three grades， namely， slight， medium， and severe， was successfully correlated with the observed degrees of corrosion. This method can therefore effectively measure the corrosion defect depth and can provide guidance for evaluating the degree of corrosion defects affecting plate structures.