Patients with extreme anxiety who are unable to cope with dental treatment under non-pharmacological behavioral management method may require sedation, or other forms pharmacological behavioral management method. The aim of this retrospective study was to investigate the effect of concomitant administration of 0.1 mg/kg intramuscular midazolam with or without sevoflurane insufflation sedation on sedation depth and cardiopulmonary function in pediatric patients. We analysed the records of anesthesia on patients who received dental treatment under deep sedation using sevoflurane insufflation from January 2013 to March 2014. Thirty-six children, aged 3 to 6 years, undergoing dental treatment were sedated using either sevoflurane insufflation alone (Group S, n = 18) or a combination of intramuscular injection of 0.1 mg/kg midazolam plus sevoflurane insufflation (Group SM, n = 18). Upon comparison, the average entropy value of group SM was lower than that of group S, but there were no statistically significant difference between the two groups (p > 0.05). The average heart rate and mean arterial pressure of group SM were higher than those of group S (p < 0.05). Concomitant intramuscular injection of 0.1 mg/kg midazolam with sevoflurane insufflation sedation is not sufficient to enhance the quality of sedation.