Abstract: A series of ionospheric modification experiments were conducted at the Arecibo Observatory to research the fluctuation of the plasma lines enhanced by the high frequency electromagnetic waves.The temporal fluctuation of the power of enhanced plasma line depends on the average heating power.When the plasma is excited efficiently,the fluctuation repetition frequency increases and the fluctuation becomes erratic and weak with the increase in the average heating power.When the peak power or the pulse length is changed,the fluctuation changes by merging or spliting of the repetition cycles.The fluctuation of ion lines is correlated with that of plasma lines,but has higher repetition frequency than plasma line.An explanation has been suggested for the observed phenomena.