OBJECTIVE To study the effects of magnesium sulfate on brain mitochondrial respiratory function in rats after experimental traumatic brain injury and the possible mechanism. METHODS The middle degree brain injury in rats was made by BIM-III multi-function impacting machine. The brain mitochondrial respiratory function was measured with oxygen electrode and the ultra-structural changes were observed with transmission electron microscope (TEM). RESULTS 1. The brain mitochondrial respiratory stage III and respiration control rate reduced significantly in the untreated groups within 24 and 72 hours. But treated Group A showed certain degree of recovery of respiratory function; treated Group B showed further improvement. 2. Untreated Group, treated Groups A and B had different degrees of mitochondrial ultra-structural damage respectively, which could be attenuated after the treatment with magnesium sulfate. CONCLUSIONS The mitochondrial respiratory function decreases significantly after traumatic brain injury. But it can be apparently improved after magnesium sulfate management along with the attenuated damage of mitochondria discovered by TEM. The longer course of treatment can obtain a better improvement of mitochondrial respiratory function.