The incidence of diabetes mellitus (DM) is increasing globally and it is a major source of concern. This study was undertaken to assess the antidiabetic effect of the aqueous extract of Moringa oleifera, Ocimum gratissimum and Vernonia amygdalina. Sixty adult Wistar rats with body weight of 120-150 g were randomly assigned to groups of five rats each (n=12). Groups 1 served as normal control; Groups 2-5 were diabetic groups; group 2 served as negative control; group 3-5 received 100, 200 and 400 mg/kg of triherbal formula respectively. The body weight (BW) and fasting blood glucose level (FBSL) of the rats were monitored weekly. At the end of the experiment, all the rats were anaesthetized intraperitoneally (I.P) and blood samples were collected by cardiac puncture for biochemical analysis. There was an increase in the BW of the control group and varying doses of tri-herbal formation. It caused 88.0% decrease in FBSL; 371.7%, 386.6% and 296.0% with respect to 100, 200 and 400 mg/kg. Sub-chronic study of the effect of the extract showed a significant increase (P<0.05) in packed cell volume (PCV), white blood counts in rat induced diabetes. The histological studies showed that the diabetic rats with the architecture of the pancreas distorted, was restored to normal by the extract. Its LD50 was found to be greater than 1000 mg/kg indicating its safety in rats. This study has shown that triherbal formula has hypoglycemic and haematogical effects.