Background: Several studies have been conducted regarding the role of clinical pharmacist in the reduction of medical costs, the impact of clinical pharmacists in internal medicine wards on cost saving had been rarely studied. Objective: The objective of this study was to define the characteristics of drug-related problem (DRPs) and evaluate the effect on cost savings and cost avoidance by clinical pharmacists in female internal medicine ward at Songklanagarind Hospital. Materials and Methods: A retrospective analysis of pharmacists’ interventions recorded for 6 months was conducted. DRPs were categorized based on Pharmaceutical Care Network Europe V9.00. The interventions that were accepted then calculated for cost saving and cost avoidance using the perspective of the healthcare institution. Results: A total of 117 DRPs were evaluated. More than 50% of DRPs only had potential of harm, were detected, and managed by clinical pharmacist before reach the patient. The 97 cases were accepted and implemented by team. The cost saving and cost avoidance were 214.26 United States Dollar (USD) and 21,697.35 USD, respectively. The highest total saving was from medication reconciliation, followed by dose adjustment. The largest cost reductions recorded were from antibiotic agents. Conclusions: Medication reconciliation and dose adjustment, especially antibiotic agents, were the major role of clinical pharmacists for cost saving.