OBJECTIVES: To compare the cost of investment between two drug dispensing systems for inpatient pharmacy units; manual system and automated dispensing system (ADM system), and find the solution for the effective use. METHODS: The cost of investment was estimated as the related cost which will incur when ADM system has been thoroughly implemented throughout the hospital. The cost of investment of manual and ADM systems were analyzed for 10-year period in the hospital perspective. Direct costs were calculated such as labor costs of pharmacist, pharmacy technician, supporting staff, capital costs, material costs, maintenance costs, and other costs. All costs were accumulated for annual cost and adjusted with 3% discount rate for the future value to present value. Sensitivity analysis was generated to find out the model which resulted in the lower net cost. RESULTS: Cost of investment over 10-years was $US 15,782,608 for manual system and $US 17,632,232 for ADM system (present system). Recently ADM system covered 220 types of tablets and ADM was not used all the time in a day in preparation, so ADM covered only 22.83% of all prescriptions. The sensitivity analysis showed that if we covered 75% of all prescriptions by ADM, cost of investment over 10 years was $US 15,737,803 thus we could save $US 44,805, $US 1,894,429 when compared with the manual system, and present ADM system, respectively. CONCLUSIONS: ADM implementation at the large academic hospital was the interesting system if we covered more prescriptions because the increasing number of prescription which using the ADM is becoming fully operation, efficiency of ADM and finally the system could reduce cost of investment. However, this study cannot assess cost saving from the decreasing medication errors, so actual saving cost of ADM system could be higher.