Abstract: A damage detection method for the cantilever beam with transverse crack was proposed. According to the theoretical analysis, damage of the beam is modeled as an additional flexibility. The relationship between the transverse displacement and the location of the damage and the additional flexibility was constructed. The ratio of the moment of inertia of the cracked section to the moment of inertia of the intact section is defined as the coefficient of the moment of inertia. A formulation for calculating the coefficient of the moment inertia from the additional flexibility was constructed using the FEM simulation. The status of damage is estimated according to the coefficient of the moment of inertia. An example about a trapezium crosssection cantilever beam was given to validate the correction of the formulation and the method.