Background The use of the bispectral index (BIS) is widespread in pediatric anesthesia, but few studies have attempted to perform a detailed evaluation of how BIS varies according to age in children under propofol anesthesia. This prospective study aimed to explore the exact relationship between BIS value and the age of 1- to 12-year-old children under propofol anesthesia. Methods This study enrolled 165 children (1 < yr. <= 12), scheduled for surgery under anesthesia, and divided them into 11 age groups. Of the 165 participants, 157 completed the study protocol. All patients were anesthetized with propofol for over 30 s. An observation period of 4 min followed. BIS values were recorded at 0 (immediately after propofol injection), 30, 40, 50, 60, 90, 120, 180, and 240 s after the injection. BIS values at each time point corresponding to the 11 age groups were compared using repeated measures ANOVA. Results BIS values significantly differed among the nine time points (p < 0.01) as well as among the different age groups (p < 0.01) after propofol administration. Post-hoc Bonferroni tests showed a difference in BIS values between groups 1-4 (1 < yr. <= 5) and groups 5-11(5 < yr. <= 12). BIS values were lower in the latter than in the former, from 50 to 240 s. The minimum BIS values in group 1 < yr. <= 5 and in group 5 < yr. <= 12 were recorded at 60 s as 49 +/- 17 and 35 +/- 14, respectively. Conclusions During propofol anesthesia, the BIS values were closely related to age, which can be divided into two groups: 1 < yr. <= 5 and 5 < yr. <= 12. BIS values of the older age group were lower than those of the younger age group at the same time points.