Economic Lot size decision is studied on this thesis foe the muti-product small batch production system. Even though economic lot size decision has been studied for the MRP system. this could be applied at the industry under the multi-product small batch production system because of very complicate and manager's lack of understand. Therefore, this technique is applied at the industry in order to minimize ordering cosy based on optimal quantity and period, and holding cost according to optimize inventory level under the muti-product small batch production system. After that, lot size decision technique is compared with lot size decision technique which has been used for analyzing and emphasizing productivity