Objective: Left atrial size can be considered an independent risk factor for cardiovascular diseases. The measurements of left atrium may be used to assess obesity, which is an important risk factor of cardiovascular diseases. It is aimed to determine the factors that effect to the left atrial size, which is an indicator of cardiac risk in obese children without hypertension. Methods: The cross-sectional observational study was performed between April 2008 and September 2009 at the clinic of Pediatric Cardiology. Eighty obese and 82 lean children were studied. Diagnosis of obesity was defined according to the World Health Organization classification as the standard deviation score of body mass index being over + 2 standard deviation of the same gender and age. All subjects underwent two-dimensional, M-mode, and Doppler echocardiographic studies. Student’s t-test, Chi-square test, Pearson correlation analysis, and multiple stepwise regression analyses were used to compare the subjects, differences in group proportions, evaluate the relation of variables with the left atrial size, and examine the effects of significant independent variables, respectively. Results: The mean age of obese patients was 11.7±2.2 years and it of lean subjects was 11.7±2.2 years. Body weight, body mass index, standard deviation score of body mass index, waist circumference, systolic and diastolic blood pressure, fasting insulin, and insulin resistance statistically increased in obese children (p<0.05). The left atrial size statistically correlated (p<0.05) with age (r=0.523), body mass index (r=0.394), waist circumference (r=0.421), diastolic blood pressure (r=0.230), insulin resistance (r=0.350), and left ventricular mass (r=0.535). It was determined that age (beta=0.491; 95% CI=0.091-0.892; p=0.001) and left ventricular mass (beta=0.055; 95% CI=0.026-0.085; p=0.017) were the most effective independent factors associated with left atrial size among other independent factors in multiple regression analysis. Conclusion: We found that left atrial size was mostly influenced by age and left ventricular mass in obese children. Therefore, it is important that left atrial size is follow up in childhood obesity.