Hot water extract of chlorella (WEC) increased the lifespan of superoxide dismutase (SOD)-1 mutant adults of Drosophila melanogaster in a dose dependent manner (200–800 µg/mL). Compounds in WEC were successively fractionated by solid phase extraction using a Sep-Pak C18 cartridge and size exclusion chromatography (SEC). Amino compounds in SEC fractions were derivatized with 6-aminoquinolyl-N-hydroxylsuccinimidyl carbamate and analyzed by reversed phased-liquid chromatography-tandem mass spectrometry. Phenylalanine, phenethylamine, isopentylamine, and 2-methylbutylamine were identified in the SEC fraction, which increased the lifespan of the D. melanogaster mutant adults. Phenethylamine, at very low doses (6–60 ng/g of diet) that roughly corresponded to those of phenethylamine in WEC (200–800 μg/mL), increased the lifespan of the D. melanogaster adults, while isopentylamine did not exert the lifespan elongation activity. Since phenethylamine did not show SOD-like activity, it did not increase lifespan by direct antioxidant activity.