Based on the introduced toroidally quadratic Lyapunov functions technique the exponential stability problem of an invariant 2-torus of nonlinear system is investigated. This problem is reduced to analysis of а boundary value decision problem for appropriate matrix differential Lyapunov equation. In connection with some projector P a concept of P-stability of linear system is introduced. Thus P-stability of the first approximation system is necessary and sufficient condition for 2-torus stability. In a three-dimensional case, the matrix differential Lyapunov equation degenerates into scalar one and its solution is reduced to the solution of some linear functional equation. From analysis of this functional equation е integral parametric stability criterion is deduced. The obtained results are used for construction of the stabilizing regulator.