Abstract. In this paper we study the existence, uniqueness and asymptotic stability of the periodic solutions for the Lipschitz systemẋ = εg (t, x, ε). Classical hypotheses in the periodic case of second Bogolyubov's theorem imply our ones. By means of the results established we construct, for small ε, the curves of dependence of the amplitude of asymptotically stable 2π-periodic solutions of the nonsmooth van der Pol oscillatorü+ε (|u| − 1)u+(1+aε)u = ελ sin t, on the detuning parameter a and the amplitude of the perturbation λ. After, we compare the resonance curves obtained, with the resonance curves of the classical van der Pol oscillatorü + ε`u 2 − 1´u + (1 + aε)u = ελ sin t, which were first constructed by Andronov and Witt.Key words. Periodic solution, asymptotic stability, averaging theory, nonsmooth differential system, nonsmooth van der Pol oscillator. AMS subject classifications. 34C29, 34C25, 47H11.1. Introduction. In the present paper we study the existence, uniqueness and asymptotic stability of the T -periodic solutions for the systeṁwhere ε > 0 is a small parameter and the function gis T -periodic in the first variable and locally Lipschitz with respect to the second one. As usual a key role will be played by the averaging functionand we shall look for those periodic solutions that starts near some v 0 ∈ g −1 0 (0). In the case that g is of class C 1 , we remind the periodic case of the second Bogolyubov's theorem ([6], Ch. 1, § 5, Theorem II) which represents a part of the averaging principle: det (g 0 ) ′ (v 0 ) = 0 assures the existence and uniqueness, for ε > 0 small, of a T -periodic solution of system (1.1) in a neighborhood of v 0 , while the fact that all the eigenvalues of the Jacobian matrix (g 0 ) ′ (v 0 ) have negative real part, provides also its asymptotic stability. This theorem has a long history and it includes results by Fatou Second Bogolubov's theorem gave a theoretical justification of resonance phenomenons in many real physical systems. The most significant example is the classical lamp oscillator whose scheme is drawn at Fig. 1.1 and whose current u is described by the following second order differential equation Fig. 3-5).where R = εR 0 , M = εM 0 , ω 2 = 1 + εb, F (t) = ελ sin t, ε > 0 is assumed to be small and the lamp characteristic is drawn at Fig 1.2a. The analysis of bifurcation