Background: Obesity as the excessive accumulation of adipose in adipocytes has still remained equivocal. Since diets contain many components to prevent from or promote diseases, assessing effects of the whole diet on health can be more practical. The purpose of this study was to investigate the association between quality of diets through healthy eating index (HEI), and body composition parameters in Iranian adults. Methods: This cross-sectional study included a total of 138 participants in the age range of 20-55 years, who referred to Endocrinology Center of Tehran University of Medical Sciences to receive health care services and fulfilled the eligibility criteria to participate in this study. Food frequency questionnaire (FFQ) was used to calculate HEI scores. Body composition data included: Fat mass (FM), fat free mass (FFM), abdominal fat (AF), muscle mass (MM), and total body water (TBW) that were collected by bioelectrical impedance instrument. Results: In this study the mean HEI score was 55.26. Based on HEI-2010 values, diet quality was good in 0.7% of participants, needed improvement in 55.9%, and was poor in 43.4% of the cases. There were significant linear trends between quality of diets and body composition parameters (P < 0.05). Conclusion: According to our study poor diet quality can be related to FM and obesity in Iranian adults.