This study aimed to elucidate the therapeutic concentration range of phenobarbital (PB) for adults, as well as the influence of therapeutic plasma exchange (TPE) on plasma concentration of PB. We retrospectively reviewed consecutive patients diagnosed with refractory status epileptic (RSE) and treated with a bolus injection of PB as well as TPE, admitted to our neurocritical care unit from November 2015 to October 2016. Continuous electroencephalographic monitoring was performed routinely for these patients. TPE was performed using a continuous-flow cell separator. PB concentrations in the plasma and cerebrospinal fluid were measured by gas chromatography-mass spectrometer analysis before and after TPE. A total of seven patients were included; among these, one patient had RSE related to anti-N-methyl-D-aspartate receptor encephalitis, another patient had Hashimoto encephalopathy, and five patients had undetermined etiology. For patients with clinical and electrographic control (n=6), the plasma concentration of PB ranged from 138 to 243.7 μg/ml. In addition, of six paired plasma samples (before and after TPE) obtained from three patients, no significant differences between the concentrations of PB were detected (P=0.6), suggesting that TPE may not significantly affect the plasma concentration of PB. This study confirmed that PB more than 100 µg/ml was effective for adults with RSE. Moreover, TPE may not have an influence on the plasma concentration of PB.