The objective of this study was to examine the efficacy of 7.5% tea tree oil (TTO) eyelash shampoo on the density of Demodex (Acari: Demodicidae) and ocular symptoms in patients with demodectic blepharitis. A total of 135 patients diagnosed with demodectic blepharitis, following clinical inspection and parasitological examinations, were included in the study. Patients were treated with eyelash shampoo, with or without TTO. Full Demodex reduction was attained in 36% of the patients who used eyelash shampoo with TTO, and the average Demodex count was reduced from 6.33/eyelash to zero (P < 0.001). In contrast, the average Demodex count in patients who did not achieve full reduction (64%) was reduced from 12.46/eyelash to 4.15/eyelash (P < 0.001). Full reduction was observed in 11.7% of patients who used TTO-free eyelash shampoo, and average Demodex count reduced from 2.00/eyelash to zero (P = 0.017). The average Demodex count was reduced from 11.98/eyelash to 7.91/eyelash in patients (88.3%) who did not achieve full reduction (P = 0.024). Additionally, ocular symptom scores (which are generated based on subjective experiences of itching, burning, the feeling of a foreign body in the eye, eye redness, and cylindrical dandruff) were reduced significantly (P < 0.001) in patients who used the eyelash shampoo with TTO. The symptom scores remained almost the same in patients who used TTO-free eyelash shampoo (P > 0.05). In conclusion, it was determined that eyelash shampoo with TTO is 3 times more effective at achieving full Demodex reduction, significantly reducing the Demodex count, and relieving ocular symptoms in patients where full reduction cannot be attained, without adverse side effects.