Myotonic dystrophy type 1 (DM1) is a multisystemic neuromuscular disease and cardiac involvement occurs in 80% of DM1 and it is the most common cause of sudden death. Few studies have illustrated cardiac involvements constituting of left ventricular tachycardia, ventricular fibrillation, conduction defects and arrhythmia. The aim of our study was to find an additional echocardiographic abnormality in the early stage of DM1 that correlate with the CTG repeat length. 21 genetically confirmed ambulatory DM1 patients were enrolled in this retrospective study. The clinical parameters included age of onset, disease duration, modified medical research council sum score (MRCSS), 6 minute walk test (6MWT). serum creatine kinase level and pulmonary function test. We examined the echocardiography at the first visit. The echocardiographic parameters such as left ventricular (LV) dimension, LV volume, LV wall thickness, LV mass index, left atrial dimension, mitral inflow velocity (E, A, E/A ratio), mitral flow deceleration time (DT), mitral annulus velocity (E', A'), E/E' ratio were measured and analyzed statistically in correlation to CTG repeat length. The asymptomatic arrhythmia was observed in 2 out of 21 patients. Among the echocardiography parameters, LV end-diastolic dimension (LVEDD), LV end-systolic dimension and mitral A velocity had significant negative correlation with CTG repeat length. Interestingly, E/A ratio showed a significant positive correlation with the CTG repeat length. The septal wall thickness also showed a significant correlation with the 6 MWT in the multiple linear regression model. The early LV diastolic dysfunction in DM1 can be partly explained by myocardial myotonia. This is the first study that showed diastolic dysfunction in early stages of DM1 and further longitudinal studies are warranted to understand the degenerative mechanism of the cardiac muscles in DM1.