Obesity is a fast-growing problem that is reaching pandemic proportions. Chlorella has many biological merits for promoting health, including detoxification, boosting the immune system, and even reversing cancer. In this study, we found that methanol extract of Chlorella reduces lipid accumulation in 3T3-L1 adipocytes. It has been postulated that these antiobesity effects could be a result of reducing adipogenesis. First, the MTT assay indicated that Chlorella significantly inhibited cell growth of 3T3-L1 preadipocytes. The accumulation of triacylglycerol in 3T3-L1 adipocytes decreased in cells treated with Chlorella versus those in untreated cells by Oil Red O staining. In parallel, Chlorella showed a significant dose-dependent increase in lactate dehydrogenase activity in culture medium of differentiated 3T3-L1 adipocytes. Second, we investigated the effects of Chlorella on the induction of apoptosis by fluorescence-activated cell sorting analysis. Chlorella showed that apoptotic cells increased in a time- and dose-dependent manner in cell apoptosis analysis by propidium iodide (PI) staining. Treatment with Chlorella decreased the number of normal cells and increased the number of apoptotic cells in a dose-dependent manner in annexin V-fluorescein isothiocyanate/PI double staining. Therefore, Chlorella is expected to efficiently reduce adipogenesis in 3T3-L1 adipocytes and to induce apoptosis in 3T3-L1 preadipocytes.