Using HPLC, the seasonal changes in the phenolic compound content of the leaves of the Gemlik olive cultivar (Olea europaea L) has been investigated with respect to the effects on the alternate bearing of the cultivar. For this purpose, the leaf concentrations of oleuropein, chlorogenic acid, caffeic acid, 3-hydroxycinnamic acid, scopolin and p-coumaric acid were analyzed at 10 day intervals around the years. The quantity and distribution of these phenolics in the leaves showed significant differences in 2008 (off year) and 2009 (on year). In the "on" year, the levels of chlorogenic and p-coumaric acids were high, whereas the abundance of other phenolic compounds was low. In contrast, during the "off" year, the chlorogenic and p-coumaric acid levels were at low levels, whereas the levels of the other phenolics were high. We found a negative relationship between the chlorogenic acid and caffeic acid concentrations in the "on" and "off" years: the amount of caffeic acid in the leaves was high, and the chlorogenic acid level was low. The contents of chlorogenic acid, caffeic acid, 3-hydroxycinnamic acid and p-coumaric acid were significantly different in the "on" and " off" years. These findings indicated that these compounds and the other phenolics examined in this study were related to alternate bearing. Interestingly, the levels of all of the phenolic compounds examined in this study were at their highest during the dormant season.