〔Abstract〕 Objective To investigate the natural evolution of the sciatic bone thickness in pediatric untreated unilateral developmental dysplasia of the hip(DDH) aged 1-15 years. Methods 329 cases of DDH children aged 1-15 years with unilateral dislocation were retrospectively recorded. The connection lines were defined on the coronal plane or axial plane of CT. The connection lines of the Y-shaped cartilage center on both sides were line H, the connection lines of the lowest edge of the ischia on both sides were line b, and the middle part of the two lines were divided into Zone 1 and Zone 2. Zone 1 represented the marginal area, and Zone 2 represented the central area. The thickness of ischium, epiphyseal plate, iliac bone thickness and epiphyseal quotient of femoral head on both sides were measured and compared. Results In coronal CL1-CL4, the ischial thickness gradually increased with age from 1 to 10 years old, and decreased from 11 to 15 years old. The range of ischial thickness of CL1-CL4 was 2.1-16.7 mm, 3.3-18.9 mm, 2.4-13.6 mm and 3.0-14.9 mm, respectively. The width of the epiphyseal plate in coronal position, the width of the epiphyseal plate in axial position, and the thickness of the iliac bone in the affected side were greater than those in the opposite side and had statistical differences. In the correlation test of ischial thickness with age and degree of dislocation, the thickness of ischial bone in coronal and axial positions was moderately correlated with age(r=0.413-0.570,P<0.05), and had no correlation with the degree of dislocation(r=0.024-0.073,P>0.05). In the correlation tests of ischial thickness and epiphyseal thickness CD, epiphyseal quotient, coronal iliac thickness IL on the affected side, the thickness of ischial bone in different parts and sections were positive(r=0.427-0.681,P<0.05), and the thickness of ischial bone was negatively correlated with the epiphyseal quotient(r=0.130-0.241,P<0.05). Conclusion The ischial thickness in coronal zone 1 and zone 2 of 1-10 years old children with unilateral DDH increased at a stable rate with age, and the growth rate decreased gradually in 11-15 years old. The thickness of ischia on the affected side in different sections and areas were greater than that on the opposite side. The difference in the central area of the hip joint was greater than that in the marginal area. The thickness of ischia was positively correlated with acetabular cartilage index, epiphyseal plate thickness, and coronal iliac bone thickness.