This Paper describes briefly a question of whether growing coral reefs could be an effective means to fix CO2 in sea water in order to mitigate global warming: Growing coral reefs could be a promising method from the following three views: Photosynthesis performed by diverse ecosystems that constitute coral reefs is thought to excel calcification (accompanying CO2 discharge from sea to the atmosphere); corals all over the world would fix CO2 at an annual rate of several hundred million tons; and no much energies are consumed to fix CO2. However, these favorable views depend upon another condition that a time period for the CO2 fixation is not too short (a measure for the time being is 100 years or longer). Elucidating where organic matters generated by the photosynthesis would go is an important question to meet this requirement. The paper indicates that measuring nitrogen and phosphor balances in addition to carbon balance is effective for measuring ratio of a net photosynthesis rate to the calcification rate. 4 refs., 1 fig., 4 tabs.