Today people can sell and buy products on online platforms, thus becoming an entrepreneur is possible for everyone. In the beginning, new entrepreneurs may be able to manage their businesses, but they will struggle with difficulties when their businesses are growing bigger. Therefore, the new entrepreneurs need to learn business knowledge to operate the businesses. The inventory management knowledge such as economic order quantity (EOQ), inventory policy, ABC analysis is one of the essential knowledge in operating a business. The stock costs or warehouse rental costs will be increased without inventory management.  This paper aims to demonstrate the importance of inventory management knowledge for operating a business. The demonstration is presented through the performance of undergraduate students in a business game called “MonsoonSim.” The students played the game before learning the concept of inventory management, then played again after learning the concept. The results showed that the average profit of retail business games improved from - 25,142.7 Baht to 2,523,461.8 Baht and the average profit of wholesale business games improved from 1,691,505.1 Baht to 2,981,308.5 Baht. The space utilization improved from -108.3 to 19.2 for the retail business games, but the space utilization decreased from 37.5 to 32.4 for the wholesale business games. Besides, days sales on inventory also improved from 54.3 to 15.3 in the retail business games while days sales on inventory improved from 75.5 to 34.5. The students were asked about their attitude towards inventory management and the game by using an online questionnaire. Most students thought that inventory management is vital for operating a business, and they can apply inventory management in their future work. Based on the result, inventory management is one of the knowledge that new entrepreneurs need to know to be able to manage and operate businesses effectively.