In this study, the effectiveness of whey protein isolate (WPI) and WPI enriched thyme essential oil coating to improve the quality of horse mackerel during frozen storage at -18 degrees C +/- 2 were evaluated for nine month. Whey protein isolate without thyme essential oil (WPIO) and three different proportion thyme essential oil (3, 5 and 7 %, v/v) in whey protein isolate (WPI3, WPI5 and WPI7) coating solutions were applied to horse mackerel. As a control, non-coated and distillated water coating (DW) horse mackerel were used. The results showed that the lowest peroxide value (PV) (meq of peroxide oxygen kg-1) and thiobarbutiric acid value (TBA) (mg malonaldehyde/kg muscle) were determined in WPI coatings enriched with 3% thyme essential oil treated group (p<0.05) while the highest PV and TBA values were determined in WPI coatings enriched with 5% and 7% thyme essential oil treated groups (p<0.05). Among the treatment, DW and the WPI coatings enriched with 7% thyme oil gave significantly higher protein solubility than other treatments during frozen storage (p<0.05). Electrophoretic studies in the presence and absence of beta-mercaptoethanol showed that high molecular weight polymers via non-disulfide and disulfide cross-linking occurred in the horse mackerel proteins extracted in 5% NaCI in all groups during frozen storage. Sensory assessment showed that the horse mackerel coated WPI enriched with 5 and 7% thyme essential oil could not be stored for more than 9 months.