The effects of H2O addition on the evolutions of soot morphology, nanostructure, and oxidation reactivity were investigated in ethylene inverse diffusion flames. The thermophoretic sampling and transmission electron microscopy (TEM) analysis were used to evaluate the soot morphological characteristics including primary particle diameter, projected area of agglomerates, and fractal dimension. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were utilized to gain the nanostructure and crystallite features and thermogravimetric analyzer (TGA) was adopted for oxidation reactivity by analyzing the soot collected via quartz plate sampling. A reduction in soot production was found with increasing H2O concentration, which illustrated an inhibition effect of H2O on soot formation. The soot morphology evolutions further demonstrated that H2O addition could decrease soot inception process from the reduced sizes of primary particles and agglomerates, and fractal dimension. The HRTEM and XRD results showed that the increasing H-2 O addition led to larger fringe length, lower fringe tortuosity, and smaller interlayer spacing of soot, which indicated a higher carbonization degree in soot nanostructure. The soot oxidation reactivity was slightly reduced with H2O addition.