The problem of the stability loss of an infinite plate with a circular insert with another material exposed to uniaxial tension is considered. The influence of elastic modulus of the insert to the value of the critical load is defined. For finding the minimum eigenvalue corresponds to the first critical load is applied variational principle. The calculations are performed in the programm “Maple” and compared with the results obtained by the method of finite elements in ANSYS. The calculations show that the loss of stability in the case when the inclusion softer than the plate, and when the inclusion more firm than plate occurs in different forms, and at the approach of the Young’s modulus of the inclusion to the Young’s modulus of the plate critical load essentially increases. In the case of the coincidence of elastic modulus of the plate and inclusion stability loss is impossible. Refs 10. Figs 5. Table 1.