Abstract. We have studied the long-term variability of solar flare index during the solar cycle 21 to 23. We noticed that the yearly occurrence of flare index varies with 11-year sunspot cycle except under some circumstances. The occurrences of solar transient events are also following solar activity cycles i.e. varies with different phases of solar cycles. We observed that the occurrence of solar flares are in the increasing trend during the declining phase of solar cycle 21, whereas the overall occurrence of flares are higher during solar cycle 21 and 22 in comparison to solar cycle 23. This phenomenon is associated with Hale cycle. During solar cycle 21 and 22 northern hemisphere and southern hemisphere of the Sun was most active. After higher value of flare index in 22-year cycle, we find the overall value of flare index is small for solar cycle 23 and we predict same condition for future solar cycle 24.