〔Abstract〕 Objective To investigate the pro-apoptotic effects of hesperidin on human lung adenocarcinoma PC9 cells as well as thevariation of proteins involved in mitochondrial pathway and endoplasmic reticulum stress(ER stress). Methods LDH Cytotoxicity Assay Kit was used to monitor the cytotoxic effect of hesperidin on PC9 cells. Fluorescence-activated cell sorting(FACS) was utilized for determining the apoptosis of PC9 cells pretreated with different dosages of hesperidin. Proteins implicated in mitochondrial apoptosis and ER stress, including Caspase-3, Caspase-9, Apaf-1,Bcl-2, cytochrome C,CHOP, Caspase-4, Bip, p-JNK, p-Perk, IRE1 and ATF6 were measured by western-blotting. Results ① Cytotoxic effect of hesperidin on PC9 cells was conspicuously aggrandized with either the increase of hesperidin concentration or prolongation of incubation time.② Apoptosis, especially early apoptosis among PC9 cells were intensified by the increasing doses of hesperidin. ③ By hesperidin administration, Caspase-3, Caspase-9, Apaf-1 and cytochrome C were upregulated and Bcl-2 downregulated. ④ Hesperidin also induced the expression of ER stress-related proteins or receptors, such as CHOP, Caspase-4, Bip, p-JNK, p-Perk, IRE1 and c-ATF6. Conclusion Hesperidin exhibited cytotoxic and pro-apoptotic efficacies on PC9 cells, which may implicate ER stress.