The measures of internal control monitoring are important in improving management and reducing the probability of internal control weaknesses for modern enterprises． This paper presents a distribution model that determines the optimal amount to invest in the internal control monitoring for modern enterprises． The model takes into account the defect rate of internal controls and the reliability of processes． The defect rate of internal controls can be decreased by investing in the internal control monitoring． Consequently，the reliability of processes can be raised． This paper utilizes the theory of reliability to measure the utility of the investment of internal control monitoring． The model proposes to maximize the utility of the firm subject to a given level of investment desired by the management． It focuses on how to allocate the amount of investment to maximize an enterprise’s utility． In a series system，if the internal control strength of each process is different，the product of the optimal investment and the internal control strength of the process is related to the probability of the error occurring． But in a parallel system，if the internal control strength of each process is different，the amount of investment is related to the internal control strength of all processes． The investment is averaged for each process，if the internal control strength of each process is the same． Further，the utility in a parallel system is higher than that in a series system．