Introduction: Some cell culture and animal studies have reported that Conjugated Linoleic Acids (CLAs) have several health related benefits. CLAs have been shown to have antiadipogenic, antiatherogenic, antidiabetogenic and anti-inflammatory properties. While increase in insulin resistance with 10-trans, 12-cis isomer of CLA was reported in some animal studies, there are controversial results about a 50:50 isomer mixture. The object of the present study was to determine the effect of CLAs supplementation (providing equal proportions of c9, t11 and t10, c12 CLA) on plasma glucose, insulin, proinsulin, C-peptide, insulin sensitivity, insulin resistance, beta cell function and HbA1c in patients with type 2 diabetes mellitus. Materials & Methods: The study was performed as an 8-week randomized double-blind, placebo-controlled parallel intervention. Participants were 39 (19 men and 20 women) type 2 diabetic subjects (35 to 50 Y, BMI >25 and <30), stratified according to sex, age and BMI into two groups. Group one were given 3.0 g CLA/d (3×1g capsules, a 50:50 isomer blend of c9, t11 and t10, c12 CLA) and, group 2 took CLA placebos (soy bean oil) for 8 weeks. Blood sample collection after fasting and 2 hours after a standard breakfast, was done before and after the intervention in order to determine insulin, glucose, pre insulin, c-peptide and HbA1c levels. Results: No significant differences were seen in fasting and postprandial glucose, insulin, proinsulin, C- peptide and HbA1c levels between groups or in insulin resistance, insulin sensitivity, beta cell function and beta cell responsiveness. Conclusion: CLA supplementation has no effects on diabetes glucose level and insulin function and its prescription is not recommended.