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This study analysed the associations of sex steroids with fat-free mass (FFM) and handgrip strength in 641 Chinese boys. Serum total testosterone (TT) and oestradiol were measured by chemiluminescence immunoassay. Free testosterone (FT) and oestradiol were calculated. FFM and handgrip strength were measured by bioelectrical impedance analysis and a hand dynamometer, respectively. Generalised additive models and multiple linear regression were used to explore the relationships. A subgroup analysis was conducted in early-mid pubertal and late-post pubertal groups. Age, height, weight, physical activity, intake of dietary protein and/or stage of puberty were adjusted. TT and FT were positively related to FFM and handgrip strength, with a curvilinear relationship being detected for handgrip strength (p<0.050). This curvilinear relationship was only observed in the late-post pubertal group, suggesting a potential threshold effect (FT>11.99ng/dL, β = 1.275, p = 0.039). In the early-mid pubertal group, TT and/or FT were linearly or near-linearly related to FFM or handgrip strength (β = 0.003–0.271, p<0.050). The association between FT and FFM was stronger than that in the late-post pubertal group. This study found that serum T had different associations with muscle parameters in Chinese early-mid pubertal and late-post pubertal boys. In the late-post pubertal boys, serum T was curvilinearly related to muscle strength with a threshold effect and its link with muscle mass was weaker. [ABSTRACT FROM AUTHOR]