Summary: PT Indonesia Asahan Aluminium (INALUM) Kuala Tanjung is an industrial company engaged in the smelting of aluminum, which will produce aluminum bar (ingot). Purchase, Sale and Inventory are some of the activities performed by the company in its operations. In these activities were vulnerable to the practice of manipulation and embezzlement. For that she needs a good internal control to monitor such activities. The purpose of this research is to get a picture of the implementation of internal control system purchases, sales and inventory implemented by the company.In writing this essay uses descriptive research design. The type of data used are primary data and secondary data, while data collection techniques used in this research is through observation, interview and documentation. Methods of data analysis used is descriptive method.The author has done the analysis and also has obtained research results of the evaluation of internal control purchases, sales and inventory in PT Indonesia Asahan Aluminium (INALUM) Kuala Tanjung, and can be drawn some conclusions from the research that has been done, namely: (a) In general the company's organizational structure in compliance with internal control, namely the existence of a clear job description of duties and responsibilities of each personnel, (b) The procedure of buying, selling and general supplies at the company's already good enough and adequate implementation, although there are some parts that must be be increased to achieve adequate internal controls in companies, (c) Documents used by the company in internal control purchasing, sales and inventories have also been quite good and adequate.