Low bone mass is one of the complications of juvenile idiopathic arthritis (JIA). However, a study focusing on the low bone mass in children and adolescents with JIA in Southeast Asian countries is limited. This study aimed to evaluate the bone mineral density (BMD) of Thai patients with JIA and identify factors correlated with BMD. A cross-sectional study was conducted at a tertiary-care center. The BMD of the lumbar spines (BMDLS) and the total body (BMDTB) were measured by dual-energy X-ray absorptiometry. Thirty-eight patients were enrolled between July 2015 and January 2016. No patient had low BMDLS, and only 2 (5.3%) had low BMDTB. Serum 25-hydroxyvitamin D (25OHD) levels were significantly positively correlated with the BMDTB Z-score (coefficient: 0.047; 95% confidence interval = 0.011-0.082; P = .012). Our study demonstrated a very low prevalence of low bone mass. Optimization of the serum 25OHD level should be encouraged.