As one way to prevent global warming due to carbon dioxide, microalgal oil production with secondarily treated sewage (STS) and thermochemical oil recovery from algal cells were studied. A hydrocarbon-rich microalga, Botryococcus braunii, produced hydrocarbons and consumed nitrate and phosphate in STS. Removal effects of the alga regarding As, Cd and Cr were observed during these experiments with an artificial medium. As for oil recovery from algal cells, the maximum yield of oil obtained by liquefaction was 64 wt% on a dry basis at 300 degrees C with a catalyst of sodium carbonate.