This paper presents the harmonic-balance-based power flow analysis (HB-PFA) method for nonlinear vibrating systems. A general power flow formulation for dynamics analysis of systems with various nonlinearities is firstly introduced. The formulation includes the derivation of energy and power balance equations, with each term representing a specific energy or power quantity. For periodic responses, approximate analytical solutions for power flow quantities are derived by the harmonic balance method. To illustrate the effectiveness of the method, a case study on the power flow of systems modelled by the Van der Pol oscillator is presented.