The aim of the present study was to investigate the effects of pre- and post- administration of garlic extract on serum glucose and some biochemical parameters in alloxan-induced diabetic rats to show the preventive and ameliorating effects in alloxan induced-diabetic rats. Rats were divided into 4 groups; normal control rats, diabetic control rats, diabetic rats post-treated with garlic extract and rats pre-treated with garlic extract before induction. Garlic extract was administered orally for 2 weeks to post-treated rats and 3 weeks to pre-treated rats and they were compared with the normal and diabetic groups, respectively. Serum glucose was reduced significantly in both post-treated and pre-treated groups. The post-treatment with garlic extract reduced serum cholesterol, but pre-treatment with garlic extract produced significant change compare to the diabetic control. The serum creatinine and urea levels were significantly reduced in post-treated group and pretreated group compare to the diabetic control group. It is concluded that the consumption of garlic produced a significant hypoglycaemic and hypolipidemic effects in diabetic rats and also, garlic is capable of protecting the liver and the kidney functions in alloxan-induced diabetic rats as shown in the activities of serum enzymes and other biochemical parameters examined.