This study was conducted to investigate the effect of lentils on serum glucose and serum lipid levels in diabetic rats. Forty adult male Sprague-Dawley rats, 12 weeks of age weighing 220-290g, were used. Diabetes was induced by streptozotocin at a level of 35 mg/kg intra-peritoneally. The animals were randomly divided into five groups, eight animals each: a casein diet (control), raw whole lentil (RWL), cooked whole lentil (CWL), raw dehulled lentil (RDL) and cooked dehulled lentil (CDL). Animals were fed with experimental diets for six weeks, sacrificed and blood samples were taken. Serum glucose level of the CDL group (387.9 ± 53.3 mg/dl) was significantly lower (P<0.05) than that of the control, RDL and RWL groups (529.0 ± 11.7, 538.6 ± 45.0, 542.1 ± 32.2 mg/dl respectively). In addition, HDL concentration of CWL group (66.3 ± 1.9 mg/dl) was significantly higher (P<0.01) than that of the control, RWL and RDL groups (54.9 ± 3.5, 50.8 ± 4.2, 54.0 ± 3.4 mg/dl respectively). However, there was no significant difference in serum glucose and serum HDL between the CDL and CWL groups. No significant differences (p>0.05) were detected in triglycerides, total cholesterol and LDL cholesterol among the experimental groups. It is concluded that cooked lentils rather than raw lentils was more effective in lowering blood glucose and improving HDL cholesterol in diabetic rats. There was no difference between whole and dehulled lentils with regard to effects on blood glucose and HDL cholesterol levels.