AbstractIn a beam‐plasma system generating a rather coherent oscillation under a quiescent and stable plasma condition, interaction between the oscillation and an artificially launched oscillation is investigated. It is shown that the latter exerts both synchronization and cross‐modulation effects on the former. An oscillator equation including nonlinear terms of the van der Pol and Mathieu equations (V‐M equation) is used to describe the phenomena. It is found that the equation gives a fair account of them, though phenomenological. Conditions for enhancement of the oscillatory energy and for noise suppression in a plasma are given.