In this study, the effect of coke particles size on sinter quality has been investigated. Different ranges of coke particles size (0-3.35, 0-2.88, 0.212-3.35, 0.212-2.88, 0.355-3.35, 0.5-3.35, 0.71-3.35mm) were chosen. Then sinter production was performed using these samples and these quality compared with the reference sample obtained from Esfahan Steel Company. Other parameters including iron ore, lime, sintering mixture particle size and suction conditions were kept constant. Thermal profiles were achieved by measuring the temperature in different regions of sintering bed. From these profiles, the maximum temperature, the amount of molten phase, the flame front speed, the solidification rate of molten phase as well as the sinter cooling rate were determined. Porosity, phase determination and morphology tests were carried out on the samples. The results illustrated that the coke size range of 0.2-3.3mm is optimum as far as the coke combustion rate, maximum temperature and the flame front speed are concerned. These lead to the rate of sinter production increase. Improved reducibility and higher mechanical strength were also obtained due to the presence of higher amounts of Calciumferrite phase with a nodular morphology. So, it can be concluded that coke particles size range 0.2-3.3mm improves the sinter quality considerably.