A previous report by the present authors [1] clarified the diffuser effect of the draft tube and the effects of swirl in the runner downstream on the hydraulic instabilities in power generation plants. The present study was intended to study the effect of finite sound velocity in the penstock, based on a similar one-dimensional analytical model as Ref. [1]. It was found that the introduction of finite sound velocity in the penstock results in the occurrence of higher order frequency modes. The diffuser and swirl effects on each frequency mode are examined in the present study.