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Background & Objective: Fetal weight and gestational age are important factors in evaluation of neonatal safety and basic prediction factors in outcome of pregnancy and satisfactory criterion in decision of management in obstetrice and neonatalogy. Therefore finding a method for pridiction of fetal weight is critical. The present study employs, detect on value of Janson's formula in prediction of fetal weight. Materials & Methods: In this analytic study, 223 pregnant women who had a safe, live, singleton fetus, with cephalic presentation were admitted to labor ward at Dezyani hospital. Medical or obstetrical complication was excluded. Subjects selected by purposive sampling. For collecting data: First, using the same flexible tap measure calibrated in centimeters by using this tap, the fundul height was measured. The fundual height was also taken in Janson's formula and estimated weight was also calculated each cases. The actual weight of post delivery was measured immediately. Data analysis was done by correlation test paired T-test. Results: This study shows mean estimated fetal weight (3730±546) were higher than the actual birth weight (3270±429). Conclusion: This study indicates that Janson's formula is not a good way to estimate fetal weight.