In this study, the effects of using different ratios of fly ash on hydration products and, mechanical and microstructural properties of hardened concrete were investigated. Portland cement was replaced with 5%, 10%, 15% and 20% fly ash (FA) by weight. Microstructural properties of obtained samples were investigated by X-ray diffraction (XRD) and Scanning electron microscopy (SEM). Fly ash has negative effect on 7 days concrete strength ; however, it was seen that fly ash increases the compressive strength of 28 and 90 days concrete. XRD analysis showed that the ratio of calcium hydroxide (CH) which is produced by hydration decreases depending on concrete age and the amount of fly ash. SEM analysis showed that the usage of fly ash decreases gaps and increases C-S-H which is also a hydration production. When Portland cement was replaced with 10% fly ash by weight, compressive strength has increased and microstructure of concrete has improved. The reason for this is filling of gaps by fly ash and the decrease in the amount of Ca(OH)2 due to the reaction between fly ash and Ca(OH)2. Within this scope, the development in microstructure of fly ash concrete was evaluated in 90 days duration and change of the development with compressive strength was investigated.