In each organization, manager is the responsible of internal control system. One of the most important internal controls that use it forerunner countries and big companies is internal audit. Internal audit can have special effects to supply the user's needs of its operation results. One of the most important users of internal audit operation results are external auditors. If external auditors know audit department and its usefulness' and decide to rely on their operation results, different aspects of external audit function will be affected, one of them is audit plan that includes different parts. This research by using analytical-descriptive approach aims to appoint kind of changes and their causes. This research results show that external auditors rely on internal auditors and their reliance causes lowering time and cost budgets in audit plan.