In this study, the effect of fly ash’s fineness on its pozzolanic activity was analyzed. For this purpose, the fly ash samples were sieved by using the screens with the apertures of 150, 106, 75 and 45 µm one by one. The unscreened original fly ash and fly ash undersize (-150, -106, -75 and – 45µm) were mixed separately with Portland cement and prepared as an additional material of concrete specimens. The concrete specimens were exposed to the curing procedure of 2,7,28 and 60 days. In order to investigate the fly ashes’ pozzolanic activities in terms of their fineness, the uniaxial compressive strength of these concrete specimens was measured and compared to control concrete specimens prepared with Portland cement. The highest pozzolanic activities of fly ash within the undersize 75 µm (-75 µm)were measured such as 82,1% and 87,4% respectively for the curing time of 7 and 28 days and 86.0% and 87.4% for the fly ash within the undersize 106 µm (-106 µm) for the same curing periods.