Glutathione S-transferases (GSTs) are phase II detoxification enzymes, which function via combining with pesticidal molecules and catalyzing the conjugation of molecules by thiol of glutathione, so as to protect tissues from oxidative stress damage. In the tick Haemaphysalis longicornis, glutathione S-transferases (HlGST and HlGST2) have been previously identified. However, the relationship between the transcription of glutathione S-transferases and the essential oil treatment in ticks remains unexplored. Hence, in the present study, the transcription profiles of HlGST and HlGST2 mRNAs were evaluated in H. longicornis after exposure to Cymbopogon citratus essential oil. At 24 h post-exposure of H. longicornis to different sublethal concentrations of C. citratus essential oil, there was significant difference (P = 0.0001) in the transcription of HlGST. Tukey's test showed that HlGST was significantly induced after treatment with 1% C. citratus essential oil (P = 0.0002); whereas no significant difference (P = 0.3551) was detected after treated by 2% C. citratus essential oil. No significant difference (P = 0.4555) in the transcription of HlGST2 between the treatment and the control group of 50% ethanol. Nevertheless, the down-regulation of HlGST2 in the treatment groups versus the untreated control group was not significant (P = 0.1208). The above results imply that the essential oil can modulate the expression of GST mRNA, therefore, further understanding of the underlying mechanisms of the GST at the molecular level could contribute to the development of effective control measures for ticks and tick-borne diseases.