Melatonin influences a number of physiological processes and is believed to play an antitumoral role in several types of cancers, but its impact on pancreatic cancer is not fully clarified. The growth inhibitory effect of melatonin on pancreatic cancer cell line SW-1990 was detected in vitro and in vivo. Annexin V/PI assay was applied to detect apoptosis and necrosis in SW-1990 cells. Changes of Bcl-2 and Bax expression were investigated by RT-PCR and Western blot. An obvious growth inhibition was found in SW-1990 after melatonin or combined treatment with melatonin and gemicitabine through both apoptosis and necrosis in vitro, and also found in transplanted tumors in nude mice. RT-PCR and Western blot showed that Bcl-2 expression was downregulated, while Bax expression was upregulated, after melatonin treatment. Melatonin may be a pro-apoptotic and pro-necrotic agent for pancreatic cancer cells via its modulation of Bcl-2/Bax balance.