The purpose of this research was to determine the effect of internal control system on operational efficiency of manufacturing firms in Nigeria. The study had four objectives and guided by four hypothesis.The target population was 40 manufacturing firms listed on the floor of Nigerian Stock Exchange (NSE). A standard multiple regression analysis was conducted to evaluate how well the independent variables were significantly related to operational efficiency (dependent variable). The work recommends the following among others: internal control managers and auditors of manufacturing firms should regularly review their control on creditors to ensure that they are ideal in order to avoid high level of bad debts.