Objective To explore the accuracy and practicability of bone age assessment for the diagnosis of idiopathic precocious puberty (IPP). Methods According to the “Gold Standard”, we selected 55 girls with IPP for the study group, and 83 normal girls for the control group. We retrospectively analyzed the first left hand-wrist radiographs at the first visit. Bone ages were assessed by using a single-blind method according to the RUS (Radius Ulna and Short bones), carpale and 20 bones method (TW2). Each had 5 decision thresholds (gt;97th percentile, gt;90th percentile, gt;75th percentile, gt;50th percentile and ≤50th percentile). The diagnostic values from RUS, carpale and 20 bones methods assessing bone age were analyzed to identify the best decision threshold. Results ① Both sensitivity and specificity of the four decision thresholds were relatively higher, including gt;90th percentile of RUS (sensitivity 0.836, specificity 0.916), gt;90th percentile of carpale (sensitivity 0.746, specificity 0.916), gt;90th and gt;75th percentile of 20 bone (sensitivity 0.746, specificity 0.964 and sensitivity 0.982, specificity 0.783, respectively). ② Area under receiver operator characteristic curve (AUR): AUR of RUS 0.939 ± 0.019 (95%CI 0.902 to 0.977), AUR of carpale 0.899 ± 0.028 (95%CI 0.845 to 0.954), AUR of 20 bone 0.958 ± 0.014 (95%CI 0.930 to 0.986). No significant difference was found (F=2.03, P=0.13). ③ Agreement assessment within-observer reliability was 89.28%, and between-observer reliability was 80.3% (Kappa 0.68, u=6.87, P＜0.01). Conclusions RUS and 20 bones methods have high accuracy for the diagnosis of idiopathic precocious puberty. Considering sensitivity and specificity, we think that ＞90th percentile of RUS is the best decision threshold.