Context: Previous studies on the association between glucose fluctuations and oxidative stress in diabetes mellitus were all conducted in adults, and most study participants had type 2 diabetes mellitus. Objective: The study objective was to examine the association of glucose fluctuation and oxidative stress in children with type 1 diabetes mellitus (T1DM) across different phases. Design: This was a case-control study. Setting: The setting was the Beijing Children's Hospital (2010-2013). Patients: Children treated for T1DM were divided into three study groups, including group A (newly diagnosed cases in the acute metabolic disturbance phase), group B (the honeymoon phase), and group C (the long-standing phase). Healthy control children were matched to the T1DM patients by age and sex. Interventions: There were no interventions. Main Outcome Measures: The 24-hour urinary free 8-iso-prostaglandin F2 alpha to creatinine (8-iso-PGF2 alpha/Cr) ratio indicated oxidative stress. Glucose fluctuation parameters (GFPs) included mean blood glucose levels, standard deviation of daily blood glucose levels, mean amplitude of glucose excursions, and incremental area under the curve for postprandial glucose. Results: In each study group, 8-iso-PGF2 alpha/Cr and all GFPs in children with T1DM were significantly higher than those in normal controls. 8-iso-PGF2/Cr was significantly correlated with all GFPs in all three T1DM groups. A stronger association with 8-iso-PGF2 alpha/Cr for mean amplitude of glucose excursions than for glycated hemoglobin was observed in both the acute metabolic disturbance and long-standing phases of T1DM. Conclusions: Glucose fluctuations were positively associated with oxidative stress in T1DM patients in different phases. Glucose fluctuations may have a stronger effect than sustained chronic hyperglycemia on triggering oxidative stress, but coexisting high levels of blood glucose may be required.