In this work, the particles of two seaweeds, Enteromorpha clathrata (E. clathrata) (EN) and Sargassum natans (S. natans), were combusted in a fluidized bed. It was found that while combustion of EN particles was stable, there was a substantial slagging period during the combustion of S. natans particles. Seaweed and its bottom ash samples were collected, and their pore structures were determined with both mercury intrusion method and N2 adsorption–desorption method. The structural analysis revealed that the number of porosity, pore volume, and specific surface area was all increased and the internal pore in ash samples was expanded after combustion. Fractal analysis showed that while the surface of original seaweed was smooth, it became irregular and rough after combustion. This study has suggested that the ash of seaweeds with porous structure can be valuable for comprehensive utilization.