How to dynamically adjust population size is one ofthe most important research topics in coevolutionary computation.This paper proposes a new strategy to dynamically adjustpopulation size during a coevolutionary process. Three factors areconsidered in the strategy to describe the influence on populationsize. They are the natural growth of sub-populations, the internalcompetition of individuals from same sub-population and theinteraction between individuals from different sub-populations. Itis proven that the proposed strategy is globally asymptoticallystable and also the convergence of a coevolutionary algorithm withthe strategy is investigated. Finally, the behavior of the proposedstrategy in practical use is illustrated.