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Carbon and nitrogen source is one of the most important factors affecting the growth ofmicroalgae. This study investigated the possibility of using oyster shells as an inorganic carbon source because it contains high amounts of carbonates and has been abandoned so much in factories. To effectively extract carbonate, the oyster shells were pretreated with acetate buffer at 45'C. The amounts of bicarbonate and dissolved carbon dioxide were measured as 1-2 %(v/v) of total amounts of the oyster shells, whose values were somewhat lower than the total carbons in the shell as 7-10%. It was also found that the available inorganic carbon sources for Chlorella sp. was a dissolved inorganic carbon source (i.e. calcium carbonate), not the all of inorganic carbons from the oyster shell as well optimal pH should be maintained during the extraction. In adding 0.5- 1 % (v/v) of oyster shell extracts, 1.94 g-dry wt./L of maximum cell density was obtained, which can be compared to 2.54 g-dry wt./L of maximum cell density with F/12 culture medium with sea water. Higher than 1.5 % of the extract would greatly reduce the cell growth possibly by containing toxic residues from the oyster shell. It can be concluded that the oyster shell is used for a cheap inorganic carbon source only by mixing with sea water even though the optimal culture condition including medium composition by using the oyster shell extracts should be further studied.