Overactivation of poly (ADP-ribose) polymerase-1 (PARP-1) has been implicated in the pathogenesis of oxidative stress-related diseases, including diabetes and its complications. Obesity is linked with type 2 diabetes. We previously reported that thiamine supplementation prevents obesity and diabetes-related liver disease. In this study, we focused on hepatic ADP-ribosylation, which reflects the activation of PARP-1. Obese diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats were randomly divided into two groups: a thiamine-supplemented group and an unsupplemented control group. ADP-ribosylated protein expression in the liver was determined by western blotting. Obese diabetic OLETF rats showed increased ADP-ribosylated protein expression in the liver. Hepatic ADP-ribosylated protein expression in thiamine-supplemented OLETF rats was lower than that in the unsupplemented control OLETF rats. These results suggest that thiamine supplementation modulates oxidative stress by inhibiting hepatic ADP-ribosylation in OLETF rats. The beneficial effect of high-dose thiamine on oxidative stress-related diseases may also be attributable to its inhibitory effect on PARP-1activation in addition to its role as a coenzyme.