PURPOSE Thymus grows after birth, reaches maximal size after the first few years and involutes by puberty. Because of the postnatal developmental and involutional duration, we aimed to investigate normal stiffness values of mediastinal thymus by shear wave elastography (SWE) in different age groups of children and discuss imaging findings of thymus. METHODS We prospectively examined 146 children (90 girls, 56 boys) who underwent a thyroid or neck ultrasound examination. All subjects underwent ultrasound and SWE evaluation of mediastinal thymus by parasternal and suprasternal approach. We grouped the subjects based on age as 0 to 2 months, >2 to 6 months, >6 months to 2 years, >2 to 5 years, >5 to 8 years, and greater than 8 years old. We investigated differences of mean shear wave elasticity (kPa) and shear wave velocity (m/s) values among age groups and the association of SWE values with age, body mass index (BMI), height, and weight of the patients. RESULTS Median and range of age, height, weight, and BMI were 24 months (2–84 months), 85 cm (55– 120 cm), 12 kg (4.55–22 kg), 15.37 kg/m2 (13.92–17.51 kg/m2), 11 cc (2.64–23.15 cc), respectively. Mean shear wave elasticity of thymus of all participants was 6.76±1.04 kPa. Differences of mean elasticity values among the age and gender groups were not statistically significant. Thymus elasticity and velocity values showed highly significant negative correlations with age (r= –0.3), height (r= –0.26), weight (r=–0.3) (P < 0.001). CONCLUSION Quantitative evaluation of the thymus by SWE provides normative stiffness values based on age and gender groups. The thymus elasticity decreases with increased age, height, and weight.