Key account management (KAM) is an approach adopted by manufacturers to build strong relationship with their key accounts. To build strong relationship manufacturers offer their key accounts tailored product/service packages on a continuing basis. Because of technological changes and emergence of larger retailers, the importance of key accounts to manufacturers are increasing. However, related research about key account management is still rare in domestic market setting. Our study focused on the effects of the manufacturers’support activities toward their key accounts. To investigate the support activities, we suggest five different support dimensions-product, incentive, service, information, human. The effects of support activities on manufacturers’ performance are measured by relationship performance and economic performance. Additionally, we examined the firm size influence on such support activities. To test our hypothesis, the data was collected from marketing and sales department personnel of manufacturers and PLS method was used to analyze the data. Our finding shows that supports related to product, services, and human have positive effects on the key account’s trust towards the manufacturers. Incentive and information supports have positive effects on the manufacturer’s economic performance. Also, our result reveals that higher level of key account’s trust towards the manufacturer has positive effects on economic performance of manufacturers. We also finds that effective support activities vary depending on the size of manufacturers. Our study contributes to the literature in the following ways. First, this research is the first attempt of trying to understand the effects of key account management in domestic market. Second, unlike previous studies which have treated support activities as one dimension, our study suggests five different dimensions of manufacturer’s support activities and try to clarify the effects of each dimension of support activities. We hope our result can provide more specific guidance to practitioners as they can understand the effects of each of their support activities.