The aim of this study was to identify the roles and potential mechanisms of endoplasmic reticulum stress (ER stress), proapoptotic transcription factor C/EBP homologous protein (CHOP) and Box in angiotensin II (Ang II)-induced cardiomyocyte apoptosis. Cultured neonatal rat cardiomyocytes were incubated with Ang II or antisense CHOP oligonucleotide which was used to inhibit CHOP expression. Expressions of ER chaperone immunoglobulin heavy chain-binding protein (BiP), CHOP and cytochrome c were examined by Western blotting. Mitochondrial membrane potential (MMP) was detected by a spectrofluorimeter. Apoptosis was analyzed with flow cytometry. Bax translocation was determined by double-labeling of immunofluorescence and Western blotting. Our results showed that Ang II-induced cardiomyocyte apoptosis was associated with the upregulations of BiP and CHOP, Box translocation, MMP deplorization and cytochrome c release. These above effects were suppressed by antisense CHOP oligonucleotide. Furthermore, BiP and CHOP expressions, reactive oxygen species (ROS) production and cardiomyocyte apoptosis, which were upregulated by Ang II, were depressed by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor apocynin. From our results, ROS, ER stress and CHOP-mediated Box translocation may be involved in Ang II-induced cardiomyocyte apoptosis.