Abstract: Managers need relevant and reliable information just in time in order to make appropriate decisions to achieve organizational goals and objectives. Financial information is one of the most important types of information. Thus sound Accounting Information System (AIS) is crucial to the firms. This study tries to find out factors affecting AIS effectiveness which is a prerequisite to improvements on organizational performance. The paper examines relationship between AIS effectiveness and Managerial Knowledge, Use of External and Internal Consultant as well as Firm's Size.