An experimental investigation was performed to study the effect of change of the IDF burner diameters on the flame characteristics (flame length, axial temperature distribution, and flame appearance) and flame emissions. The study was performed using a coaxial (CoA) circular inverse diffusion flame burners of different diameters. Burners' air diameters (da) were varied from 6 mm to 22.5 mm. The burner fuel diameters were varied according to the change of air diameter to preserve constant equivalence ratio and constant aspect ratio (0.5). The study is carried out at constant equivalence ratio (Phi = 3) and constant air Reynolds number (Re-a = 2500). The measured parameters are the flame axial temperature, flame appearance, and emission along the flame center line. The results show significant differences in the flame appearance. The smaller diameters produce shorter flame lengths. The visible flame lengths were varied from 170 to 300 mm. Also, an early formation of CO & CO(2)and early depletion of O-2 were obtained by reducing the burner's sizes. For all the experiments conducted in the present study, the centerline temperature distribution produced by the smaller nozzles diameters show higher flame temperature. The Peak temperatures were varied according to burner size, from 1600 to 950 degrees C. (C) 2018 Elsevier Ltd. All rights reserved.