The phenolic compounds, total phenolic content, and antioxidant activity of Gemlik variety olive fruits obtained from four different locations in Turkey on five different harvesting dates (HDs) were investigated. The determination of phenolic compounds in olive flesh methanolic extracts was done by high performance liquid chromatography (HPLC) analysis. The total phenolic content and antioxidant activity values were estimated using Folin-Ciocalteu reagent and 2,2-diphenyl-1-picodrylhydrazyl reagent methods, respectively. Among 11 phenolic compounds identified, vanillic acid and hydroxytyrosol are the most abundant ones. In general, olive fruits from the Umurbey location had higher levels of phenolic compounds. The antioxidant activities of the olives were related to their phenolic content, and the olive fruits from the Umurbey location had a mean antioxidant activity 66.52% higher than that of other olives. There were differences among the quantities of phenolic compounds, the total phenolic contents, and the antioxidant activities of the olive fruits depending on HDs and location.