The aim of this study was to determine the effect of peppermint and thyme essential oil mist on performance, blood parameters and lymphoid organ indices in broiler chickens. The experiment was conducted on 360 broilers raised until 42 days of age. One-day-old chicks were divided into three groups (n = 120): control group (C – misted with pure water) and two experimental groups (PO – misted with peppermint essential oil, and TO – misted with thyme essential oil) and placed in three separate experimental rooms. Growth performance (BW – body weight, WG – weight gain, FCR – feed conversion ratio), blood parameters (RBC – red blood cells, HGB – haemoglobin concentration, HCT – haematocrit levels, leukogram, serum protein and protein fractions), and lymphoid organ indices were assessed during the production cycle. Our results demonstrate that the use of EO mist does not exert adverse effects on broiler health, and that PO mist can increase BW and WG in broiler chickens. Mean RBC parameters at slaughter were higher in broilers exposed to PO mist. An analysis of blood parameters and lymphoid organ indices revealed that EO could positively influence the immune systems of birds. Higher values of gamma-globulin concentration as well as the weights and growth indices of lymphoid organs were noted in experimental groups.