This study is based on an experimental platform for coaxial laminar diffusion flames, employing thermophoresis probe sampling and high-resolution transmission electron microscopy. It investigates the micro- and nano-morphology of carbonaceous particles in ethylene/methanol coaxial diffusion flames under varying methanol substitution rates. The micro-morphology focuses primarily on analyzing the size and number of carbonaceous particles, while the nano-morphology examines changes in the oxidative properties of these particles through the analysis of carbon layer length, interlayer spacing, and carbon layer curvature. The results reveal that with increasing height above the burner (HAB), the ethylene flames mixed with different proportions of methanol exhibit trends in which the number, size, fractal dimension, carbon layer curvature, and interlayer spacing of carbonaceous primary particles initially increase and then decrease, while carbon layer length continues to rise. Compared to pure ethylene flames, in flames at a methanol substitution rate of 60%, the growth rate of soot particle size at the flame front is lower, and the particle size decreases faster towards the flame end. As the methanol substitution rate increases, particles collected at a flame height of 15 mm gradually shift from aggregated carbonaceous particles to isolated primary soot particles, with pure methanol or high methanol ratio diffusion flames producing almost no soot. In terms of micro-morphology, as the methanol substitution rate increases, laminar diffusion flames show a gradual decrease in the number, size, and fractal dimension of primary particles produced. This trend is primarily due to the higher methanol substitution rate, which enhances the total oxygen content in the fuel, leading to more complete combustion and thereby inhibiting the formation of soot precursors, as well as the nucleation and growth processes of carbonaceous particles. In terms of nano-morphology, under pure ethylene conditions, soot particles exhibit a nearly spherical shape with a shell-like structure composed of a graphitic outer shell and an amorphous inner core. Conversely, under high methanol substitution rates, the internal carbon layers of soot extracted from the flame appear more loosely packed and exhibit greater curvature. Increasing methanol substitution rates result in a continuous decrease in carbon layer length and an increase in carbon layer curvature and interlayer spacing, indicating heightened oxidative properties of soot. In conclusion, this study provides detailed insights into how varying methanol substitution rates affect the formation and structure of carbonaceous particles in ethylene/methanol coaxial diffusion flames, at both the micro- and nano-scales. The findings underscore the complex interplay between fuel composition, combustion dynamics, and soot morphology, contributing to our understanding of particulate emissions in alternative fuel combustion systems.