Orderly growth of mesophase was studied using FCC slurry as raw materials, in a multi-tube well-crucible furnace controlled by programmed heating. The thermal decomposition gas flow was continuously observed to see its influence on the mechanism of mesophase orderly growth using a high temperature optical microscope. It is shown that reactant yield was rapidly decreased in the initial reaction stage and tended towards stability gradually on a later reaction stage, and the yield could be represented by an index decay function. The polymerization initial stage was the main period of formulation and growth of mesophase in the pipe of variable diameter, and the size distribution of mesophases was non-uniform. With the growth of mesophase and the increase of its content, the viscosity was also increased in the system. The orderly growth of mesophase was promoted by the single flow of the thermal decomposition gas. In-situ observation shows that there was a little micro-crystal in the raw materials, while it was precisely the superfine anisotropic crystal that further promoted the growth and development of mesophase. The suitable temperature for mesophase growth was in the range of 440~450℃. In addition, the mesophase moving path was observed as in a continuous change in the high temperature optical microscope because of surface tension and the side blowing of N2. That fact further verifies the influence of the flow of gas on orderly growth of mesophase.