In this research, measurement of serum blood glucose of cultured rainbow trout (Broodstocks) had done in two different treatments. First treatment consists of fish which were fast, and second treatment had feed. Each treatment includes 10 fish (50% male, 50% female). Blood extracted from caudal vein of each fish by a syringe. Then, serum separated with centrifugation. The amount of serum glucose measured by a kit (Man^® Company, Iran). The kit worked based on glucose oxidase method. The average of blood glucose in fasting treatment, was 92.2±14 mg/dl while the average of feed treatment was 189.2±17.2 mg/dl. Statistical analysis had done by SPSS software. Comparison of mean blood sugar concentration between two treatments by t-test showed significant difference between two treatments (p<0.05). The final conclusions showed that fish feeding gives change in blood glucose concentration. It is appeared that, bleed of fish for measurement of the serum glucose needs to do 8 hours after feeding for future research and accuracy tests and standardization of tests.