The hydrological regime on the river systems in Vietnam is very different. Flow frequency in a same year among river systems does not occur similarly, but has phase difference. Mean-while, the firm energy- o f hydropower stations is calculated from viewpoint as they work individually. Thus, when using these parameters to balance the energy o f the whole power system, it is considered a completely extreme case, or considered that hydrological conditions with the same flow frequency appear simultaneously in all hydropower stations in same year. This article presents the scientific basis, thereby providing a calculation method to evaluate the influence o f phase different hydrological regime on the capability- o f the hydropower source when participating in the energy’ balance o f the power system. Results from applying calculations for 18 hydropower stations on 05 river systems clearly demonstrate the influence o f phase different hydrological regime.