Background: Hypertrophic cardiomyopathy (H- CM) is hereditary cardiomyopathy that is characterized by left ventricular hypertrophy (LVH). LVH can develop in response to some factors, such as hypertension, aortic stenosis, or diffuse myocardial fibrosis (DMF). Cardiovascular magnetic resonance (CMR) has been widely used to quantify DMF by calculating the extracellular volume fraction (ECV) of the myocardium. Our aim was to investigate whether the severity of ventricular hypertrophy is associated with the degree of DMF in patients with HCM. Materials and methods: Patients with HCM (n = 125, age 58 ± 16 yrs) and Brugada syndrome (Brs) which was considered as the relative normal heart control (n = 58, age 51 ± 14 yrs) were retrospectively enrolled in this study. Pre- and postcontrast T1 maps and Cine images were acquired at 1.5 T for the assessment of ECV and wall thickness (WT). ECV and WT measurements at 4 different regions, including the anterior, septal, posterior, and lateral wall regions, were measured, and all groups were compared. Regional ECV was correlated with the corresponding regional WT. Group comparison between two groups in ECV and WT were analyzed with the Mann–Whitney U test. Linear regression was used to obtain a correlation coefficient between regional ECV and WT. The statistical tests were two-tailed, and statistical significance was defined as p < 0.05. Results: For the group comparison, HCM patients showed a significantly higher WT compared to that in controls (11.3 ± 5.1 mm vs. 6.0 ± 2.2 mm, p < 0.001). In addition, HCM patients also revealed significantly higher ECV compared to that in controls (27.5 ± 4.9% vs. 25.0 ± 2.3%, p < 0.001). The regional ECV was significantly correlated with the corresponding regional WT in patients with HCM (r = 0.311, p < 0.001). In contrast, no significant correlation (r = 0.139, p = 0.297) was found between regional ECV and WT in controls. Conclusion: In this study, we demonstrated that HCM pa tients with higher WT compared with controls. This finding supports that HCM patients have significant LVH. The increased ECV was positively associated with the corresponding regional wall thickness. This finding suggests that the severity of LVH might be associated with the degree of DMF in patients with HCM.