In this paper is investigated the solvability of a periodic problem for a system of nonlinear ordinary differential equations second order with the main positively homogeneous part. New conditions have been found that provide an a priori estimate solutions of the periodic problem under consideration. The conditions for the a priori estimate are formulated in terms of the properties of the main positively homogeneous part of the system of equations. Under the conditions of an a priori estimate, using and developing methods for calculating the mapping degree, a theorem on the solvability of the periodic problem is proven. The proven theorem generalizes previously obtained the authors’ results on the study of a periodic problem for systems of nonlinear ordinary differential equations of the second order.