Abstract: The present investigation utilized primary data collected from 383 walnut growers in provinces of Semnan Hamadan and Fars in Iran. The Cob-Douglas production function was used to test the factors effective on walnut (Juglans regia) production. The variables of this study were Zulonfloo poison, labor, machinery, Iron fertilization, water and acreage. The Cob-Douglas production function selected as the most appropriate model to analyze the walnut production function. Econometric analysis results revealed that walnut growers have used the factors of production in the second area of production and human labor, farmyard manure, chemical fertilizers, water for irrigation and transformation contributed significantly to the yield. The result showed that there is increase of returns to scale in walnut orchards of Hamadan, Fars and Semnan Provinces. The findings also showed that the elasticity of factors production such as Zulonfloo poison, labor, machinery, Iron fertilization, water and acreage were 0.810, 0.169, 0.097, 0.212, 0.158 and 0.093, respectively. Finally, the result of Wald test showed that there is an Increase of Returns to Scale (IRS) in walnut orchards of Hamadan, Fars and Semnan Provinces. Returns to scale is 1.226 means Increase of one percent of all production factors simultaneously causes 1.226% increases in product.