Subsets of mitochondrial transfer RNA (tRNA) contain the N-6-isopentenyladenosine (i(6)A) or 2-methylthio-N-6-isopentenyladenosine (ms(2)i(6)A) modification at position A37, which is adjacent to an anticodon. These modifications are essential for efficient protein translation in mitochondria and contribute to energy metabolism. The first step in i(6)A and ms(2)i(6)A modifications is catalyzed by tRNA isopentenyltransferase, which is encoded by the TRIT1 gene. Herein, we report a girl with a developmental delay, frequent episodes of seizures induced by febrile illness, and myoclonic epilepsy who had compound heterozygous missense mutations in TRIT1. A mass spectrometry analysis of RNA nucleoside obtained from the subject's peripheral blood and urine showed a marked decrease in both i(6)A and ms(2)i(6)A modifications. These results suggest that the mitochondrial disorder was caused by defective tRNA isopentenylation arising from a loss-of-function mutation in TRIT1. Furthermore, the present observations suggest that noninvasive biochemical analysis using peripheral blood and urine samples are sufficient for the diagnosis of TRIT1-related disorders, making muscle biopsy for the direct measurement of oxidative phosphorylation unnecessary. Such biochemical analyses before the start of antiepileptic medications would be beneficial to avoid hepatotoxicity in patients with possible mitochondrial disorders.