The aims of this study were to investigate the correlations of tri-nucleotide (CTG) repeat length with detailed echocardiography (ECHO) parameters that represent myocardial function and to find a relationship between heart function and CTG repeat length in adult-onset myotonic dystrophy type 1 (DM1). In this study, clinical data for patients with DM1, including age, onset age, CTG repeat length, Medical Research Council sum score (MRCSS), and 6-min walking test (6MWT), were recorded. In addition, ECHO parameters and cardiac conduction abnormalities were evaluated. Among the cardiac parameters, the EA ratio and left ventricular end-diastolic dimension (LVEDD) were significantly correlated with the CTG repeat length (p < 0.05). Interventricular septal thickness at end-diastole was also significantly correlated with the 6MWT in a multivariate linear regression model (p < 0.05). In conclusion, motor function (MRCSS and 6MWT) and CTG repeat length significantly correlated with LV diastolic dysfunction in patients with DM1. More emphasis should be given to diastolic dysfunction, which is currently under-recognized, when evaluating patients with DM1 with no abnormalities in routine electrocardiography studies. Lastly, well-designed and longitudinal studies are warranted to characterize and understand the pathophysiology of diastolic dysfunction in DM1.