Background:With recognition of the important roles of Vitamin D (VitD) in various physiological processes,increasing attention has been drawn to the status of VitD in early life.However,the VitD status of young children and the related factors in rural areas of Southwest China remain unclear.This study aimed to explore VitD status and its seasonal variation in 18-month-old children living in rural Southwest China.The association of VitD with biochemical and anthropometric variables was also investigated.Methods:A total of 177 18-month-old children in a rural area of Yunnan Province,Southwestem China,were enrolled.Serum concentrations of 25-hydroxy Vitamin D (25(OH)D) were measured through high-performance liquid chromatogram-tandem mass spectrometry.Parathyroid hormone (PTH) levels were measured with a chemiluminescence assay.Serum concentrations of calcium,phosphorus,and alkaline phosphatase (ALP) were also measured.Anthropometric data and the outdoor activity time of each participant were collected.Results:The serum 25(OH)D concentration was 26.61 ± 7.26 ng/ml;concentrations lower than 30 ng/ml accounted for 70.6％ of the participants and concentrations lower than 20 ng/ml accounted for 16.4％.The level of serum 25(OH)D was not significantly different among four seasons (P ＞ 0.05).A positive relationship was found between 25(OH)D concentration and the time of outdoor activities (r =0.168,P ＜ 0.05).Serum PTH concentration was negatively correlated with 25(OH)D concentration (r =-0.163,P ＜ 0.05).A positive relationship was found between the serum concentrations of 25(OH)D and calcium (r =0.154,P ＜ 0.05).No significant association was observed between 25(OH)D and ALP,phosphorus,or anthropometric variables.Conclusions:The prevalence of VitD insufficiency is high among young children in the rural Southwest China regardless of the seasons.VitD supplementation is still essential to maintain VitD sufficiency for children living in rural area.