This paper presented a method for modal analysis of Euler-Bernoulli beam with open cracks based on perturbation method. Beam cracks are firstly modeled as the cross section reduction of the cracked segment. The moment of inertia and mass per unit length of the beam are then expressed using function. The equation of motion for the cracked Euler-Bernoulli beam is formulated. Based on perturbation theory, beam natural frequencies and mode shapes are expressed using the first order perturbation terms. Case studies on a simply supported beam and a cantilever beam tell that the presented method is of good precision to match the results of finite element simulation and experimental tests. Based on these formulations, the effects of crack size and location on the variation of beam modal parameters are analyzed. The results tell that although micro-crake will induce small variation on structural natural frequencies, the variations of multiple modes are of a pattern to indicate crack location. The mode shape is not sensitive to the size and location variation of crack. However, the curvature mode shape is quite sensitive to indicate the location and relative severity of cracks.