In initial sintering stage of the porous (porosity≥50%) matrix impregnated fully with molten carbonate (46wt% electrolyte) and sintered under the cell stacking pressure, the sintering brought about the rearrangements and slips of powder particles by which the maximum pore diameter altered into a smaller one. Its maximum pore diameter and slip rate were smaller and oppositely higher respectively than those in the matrix impregnated with the less (5wt%) electrolyte and sintered under the ordinary pressure. The cell stacking pressure facilitated the rearrangements and slips of powder particles, but effectively suppressed and diminished defect developments in the sintered matrix. Porosity in the matrix increased, micro-pores altered into bigger ones, mean pore diameter increasing and pore-size distribution becoming narrowly with sintering time by the synergetic action of the rearrangements of powder particles and its accompanying mechanisms. As indicated above, the sintering behaviors and the alteration of some physical properties with sintering time in the porous matrix are different from those in traditional ceramics. So the rearrangement of powder particles dominates over the sintering mechanisms of the porous matrix, its sintering model conforms to the trapezium model.