The authors discuss a method of approximate calculation of possible values of vibrodiagnostic parameters of the presence of a breathing crack of different crack front configurations (semielliptical, semi-circular, or straight one) in a round bar under super- and subharmonic resonances in a natural bending mode. The calculation of the vibrodiagnostic parameters due to the presence of a crack has been performed through the example of a cantilever bar in the case of the resonance in the first normal mode under force and kinematic excitations. The vibrodiagnostic parameters have been found as a function of the place of the exciting force application, crack location, relative depth and relative area, for different ellipse semi-axes ratios and for cracks with straight or semi-circular fronts. A comparison is made between the calculated results and the finite-element numerical solution data.