The morphologies of soot deposit on 15 μm diameter silicon carbide (SiC) fibers have been investigated with a scanning electron microscope (SEM) in methane-acetylene/air laminar diffusion flames with co-flowing air. The morphologies are shown to be strongly dependent on the fuels ratio. Two kinds of processes by which mature soot particles are produced were proved to exist in a sooting flame: one is the transition from the condensed-phase deposits; the other is the aggregation of the smaller soot particles (or chains of them) carried along the particle path line. Different transition processes are compared between the present work and previous work done by other researchers that usedpropane/air laminar diffusion flames. It seems the presence of C=C in methane-acetylene laminar diffusion flames is the key factor that causes the difference of transition processes in those two kinds of flames.