$Ca^{++}$ content and histochemical changes during processing Mooks prepared with sea mustard (Undaria pinnatifida) and sea tangle (Laminaria japonica) were studied. $Ca^{++}$ content of sea mustard Mook during gelation in $CaCl_2$ solution was not only increased fast, but also decreased fast during soaking in distilled water as comparing with that of sea tangle Mook. Both solubilization of the seaweeds by 1 % $K_2HPO_4$ and filtration of solubilized seaweed were decreased the extruding amount of $Ca^{++}$ from Mooks during soaking. The cell walls in sea mustard were likely irregular and ling, but those in sea tangle were regular and round shape. alginic acid was mainly occurred around cell walls. The alginic acid was gradually solubilized depending on heating and alkali treatment. Thus, the fillament cells in sea tangle were cut finely during processing Mook, while those in sea mustard were remained tolerably.