The purposes of this research were to study demand forecasting and production planning. The case study company manufactured the fish sauce product that was made to stock. To design the system the sale data was studied in order to choose the appropriate forecasting technique for the sale data. After using the system with the company case study it revealed that the prediction system can forecast the production of 1126570 units in July 2008 whereas the total sales were 1134322 units. The difference was equal to 2718 units or 0.69 percent of the actual sales which was a total cost of 116280 baht which was more equivalent to the actual sales predicted by a sole experienced decision maker who predicted the production at 1274098 units. The difference was equal to 139776 units or 10.97% percent of the actual sales which mean 2096640 baht of total cost