Introduction: With the incidence of Type 1 diabetes mellitus (T1DM) under age of 15 is increasing worldwide, the successful healthcare transition from pediatric to adult has become aware of the need to maintain the continuity of care. Yet little is known about current status regarding healthcare transition for patients with T1DM in Korea. The purpose of this study was to investigate associations of healthcare transition readiness with family support and self-management competency in Korean emerging adults with T1DM. Methods: A cross-sectional survey was conducted between May, 2 and June 4, 2017. Patients (n = 87) ages 16 to 24 were recruited from the outpatient clinic of Severance children’s hospital and online websites for patients with T1DM. Family support was measured by 16 questions in two domains including therapeutic and emotional support. Self-management competency was assessed across nine domains including glycemic control, insulin injection, dietary, hygiene, exercise, rest, regular check-up, hypoglycemia handling and hyperglycemia handling. Healthcare transition readiness was measured by the STARx Questionnaire in six domains including medication management, provider communication, engagement during appointment, disease knowledge, adult responsibilities and resource utilization. Results: The scores of healthcare transition readiness in all domains, except for disease knowledge and medicine management, were lower than the grand mean of the total scores. Healthcare transition readiness was positively correlated with family support (r = .257, p = .016) and self-management competency (r = .606, p .001). A score of therapeutic support by family was lower than that of emotional support. Among 9 domains of self-management competency, the score of hypoglycemia handling domain was the highest. Yet multivariate linear regression showed that only self-management competency was a significant factor associated with healthcare transition readiness (β = .699, p .001). Conclusions: For the successful healthcare transition in emerging adults with T1DM, it is necessary to strengthen all domains of healthcare transition readiness, except for disease knowledge and medicine management. Moreover, improving the self-management competency would be effective to enhance the healthcare transition readiness.