Fish is one of the domestic products that is unmatched in terms of protein and unsaturated fat, and its importance as a healthy food is known to all. Keeping in mind, resource scarcity, economic method of increasing this product is the optimum use of the factors of production. The aim of this research was to estimate production function and calculation of productivity of warm water fish production factors in Kermanshah province. The statistical population in this reaserch were all of active, fish farms which were studied in full details. A part of required data of this research was collected from official statistic books of government and another part from interview with farmers and experts of fishery department of Kermanshah through distribution of 46 questionnaires. The production function has been estimated with help of Eviews and with OLS approach and through priority model of Cobb-Douglas was finally selected. Result showed that area under cultivation (land), concentrate and maize, fodder, lime stone, labour, experience of manager of the farm, use of air purifier machine and geographical condition of the region had significant effect on production (p<0.05). Total coefficient of independent variable in estimated production function was approximately 1.1 which was an indication of increasing return to scale. Elasticity of production of area under cultivation was more than other inputs which indicated that the area under cultivation must expand and more area go under cultivation of this product. In addition coefficient of labour was estimated 0.31 which indicated more employment of labour force must be done in order to approach towards optimum product. According to obtained results of this research, education did not have a significant effect on production, while experience had a positive and significant effect on it and this showed that the level of technology in this industry was not so high. The finding also showed that geographical region and use of air purifier had a significant effect on the production of warm water fish. Therefore on this base the farms under review were divided into three categories and the average productivity, marginal productivity and total factors productivity were calculated for each one of these categories. Just in small number of farms, related to one of the categories the measure of VMPxi / Pxi for descriptive variable was equal to 1 and for other two categories this measure was not equal to one. Which means that the inputs were underemployed? Therefor it seems in order to improve efficiency in input consumption, the fish farm owner can attain maximum profit by using substitution principle.