The medicinal properties of Cinnamon cassia (C. cassia) bark have been reported for their clinical importance for many diseases including diabetes. However, there is no clear evidence so far regarding dose selection for its hepato- and nephroprotective effect in diabetic condition. Hence, the present study aims at evaluating in vitro antioxidant activity, the acute toxicity, and dose fixation of C. cassia bark for their effective medicinal values in streptozotocin (STZ)-induced rats. All the extracts exhibited potential in vitro antioxidant activity and showed a dose-dependent (1000, 2000, 3000, 4000, and 5000 mg/kg BW) acute toxicity by in vivo model. The levels of aspartate transaminase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP), urea, and creatinine showed a significant elevation in animals treated with the highest dose. In further studies along with histopathological studies, animals treated with STZ (60 mg/kg BW) followed by a different dose (300, 400, and 500 mg/kg BW) of ethanolic extract of the C. cassia bark and glibenclamide (3 mg/kg BW) revealed that the altered level of mitochondrial enzymes, hepatic, and renal marker in STZ-induced animals were restored in C. cassia bark extract-treated group as of control. These results could be of scientific support for the use of the ethanolic extract of the C. cassia bark in folk medicine for the management of diabetes and its associated complications.