Early life adverse events in children orphaned by parental HIV/AIDS can have long-lasting effects on brain function. However, data are limited regarding the brain activation during resting state in these children. The current study aims to investigate the brain oscillation at rest in children orphaned by HIV/AIDS using resting-state EEG data. Data were derived from a larger neurodevelopmental study in which 90 children aged 9–17 years orphaned by AIDS and 66 matched controls were recruited through the local communities and school system. Children (63 orphans and 65 controls) who had resting-state EEG data and completed the Teacher-Child Rating Scale (T-CRS) were included in the current analysis. EEG results showed that children orphaned by HIV/AIDS had increased theta activity in the midline region, decreased beta activity in left hemisphere and overall increased theta/beta power ratio. Furthermore, the theta/beta ratio is positively correlated with learning problem and acting out scores, and negatively correlated with task orientation and peer social skill activities. Findings support that children orphaned by HIV/AIDS demonstrate different brain activity compared to non-orphan controls and also suggest that the EEG activities in resting state may serve as useful indicators of children’s behavioral problems.