The mountainous duskywing, Erynnis montanus, belongs to a lepidopteran family Hesperiidae. The 15,530-bp long complete mitochondrial genome (mitogenome) of the species has the typical gene content of animals (13 protein-coding genes, two rRNA genes, 22 tRNA genes and one major non-coding A+T-rich region). As typical in lepidopteran mitogenome E. montanus mitogenome also contained a high A/T content in the whole genome (81.7%) and the CGA (arginine) as the start codon for the COI gene. Unlike other lepidopteran species, including two sequenced skippers, the E. montanus mitogenome has a unique arrangement tRNA(Ser)-tRNA(Asn), instead of the tRNA(Asn)-tRNA(Ser) found unanimously in other lepidopteran species, providing a new gene arrangement in Lepidoptera. Such rearrangement probably was likely caused by duplication of gene block tRNA(Ser)-tRNA(Asn) and subsequent random loss of tRNA(Asn) in the first copy and tRNA(Ser) in the second copy, resulting in the arrangement tRNA(Ser)-tRNA(Asn).