Background: Current trends in Korea population aging with advances in public health and clinical medicine foretell rises in the prevalence of not only chronic diseases but also patients with multimorbidity. One important aspect in analyzing multimorbidity is to define the list of chronic diseases included when calculating multimorbidity index. The objective of this study is to describing the effect of multimorbidity on healthcare cost in Korea using US Office of the Assistant Secretary of Health (OASH) list. Methods: We analyzed the Korea Health Panel Data representing non-institutionalized Korean adult populations aged 20 and more. We calculated multimorbidity index based on OASH list and estimated the prevalence and healthcare cost for each OASH chronic disease. Results: In 2011, 15.2 million (39.6%) Koreans aged 20 and more were living with chronic condition. The health care cost due to chronic diseases, accounted for 80.2% of the overall healthcare costs and the prevalence of chronic conditions, the prevalence of multimorbidity and healthcare cost increased with ages. In the analysis using OASH list, 40% of the adult population over the age of 20 and 66.7% of the population over the age of 65 was affected with multimorbidity. In most of diseases in OASH list, prevalence of mulitmorbidity was high and healthcare cost increased with multimorbidity. Conclusion: OASH chronic disease list that accounts for 72.4% of prevalence and 86.7% of healthcare cost of persons with chronic conditions in Korea. OASH chronic disease list would be a useful and representative indicator for studying multimorbidity.