In this study, the effect of main and side chain lengths of polycarboxylate ether-based high-range water-reducing admixtures (WRAs) on the fresh properties, compressive strength and water absorption of cementitious systems containing 0, 15, 30 and 45 wt% fly ash was investigated. For this purpose, three WRAs with the same molecular weight but different chain lengths were produced. According to the test results, the flowability of the paste and mortars was negatively affected when the length of the main and side chains of the admixture was longer or shorter than a certain value. This adverse effect is thought to arise from weakening of the adsorption of the admixture with an increase in its chain lengths. However, when the main and side chain lengths of the admixture were shorter or longer than a certain value, the time-dependent flow properties of the mortar mixtures improved. The main and side chain lengths of the WRAs did not have a significant effect on the compressive strength and water absorption capacity of the mortar mixtures. However, irrespective of the admixture characteristics, with an increase in fly ash substitution, the flow and time-dependent flow properties of the mixtures were negatively affected but their water absorption decreased.