Background and Objectives Kawasaki disease (KD) is the leading cause of acquired heart disease which predominantly occurs in children younger than five years old. But, there are fewer cases in infants younger than six months which makes difficult to diagnose. We investigate the clinical manifestations of this age group and compare them to older age group of Kawasaki disease. Materials and Methods We retrospectively reviewed the medical records of 239 patients with KD admitted at the Hallym Sacred Heart Hospital from January 2013 to June 2015. The data were categorized into two groups by age: group A (≤6 months, n=26) and group B (＞6 months, n=213). Results Group A have longer hospitalization period than group B (6.69 vs 5.19 day, p 0.002). Group A has fewer clinical manifestation at admission which makes higher incidence of incomplete Kawasaki disease (1.88 vs 3.54 of diagnostic criteria, p 0.05; 19.2% vs 4.2% of incomplete Kawasaki disease incidence, p 0.002). The rate of cardiac complication was higher in group A (30.8% vs 11.7%, p=0.011). Group A patients have significantly higher white blood cell and platelet count and lower hematocrit. Conclusion The rate of cardiac complication of patients younger than 6 months of age was significantly higher than older patients. However, the diagnosis of KD in infants is difficult and delayed because they do not have all the classic manifestations. So, the echocardiography should be considered in infants younger than 6 months of age with fever of over than 5 days duration, if any principal clinical features, and no other explanation for the acute febrile illness.