This study compared the effects of herbal essential oils, organic acids, and medium-chain fatty acids, and their sole and decreased mixture on performance, egg quality, blood constituents, and fecal microbial activity in laying hens. A total of 162 Hy-line W80 38-week-old laying hens were allocated into 6 treatment groups with 9 replicates. The treatment groups were as follows: 1: Control (C, basal diet, no supplement), 2: Supplemented with a herbal essential oil mixture (EOs), 3: 400 mg/kg humic acid (HA), 4: 15 mg/kg medium-chain fatty acids mixture (MCT), 5: EOs + HA + MCT mixture (EHAM), 6. A decreased mixture of EOs + HA + MCT (EHAM-L, 1/2 of EHAM). The results showed that the diet additives did not have a significant effect on the final body weight (BW) of hens (P<0.05). The feed conversion ratio (FCR) was significantly higher in the control compared to the supplemented groups (P<0.01). The egg yield (number and percentage) and the egg mass in supplemented diets were greater than those of the control group (P< 0.01). The Haugh unit, Roche color scale value, eggshell ratio, eggshell surface area, and eggshell unit weight did not show significant differences between groups. During the whole period, eggshell thickness in all supplemented groups was higher than that of the control group (P < 0.01), and eggshell weight in the EHAM-L group was greater than in the control group (P<0.01). Diet additives did not have a significant effect on egg albumen height and yolk color. Although diet additives did not affect blood parameters, they decreased fecal bacteria counts, such as Escherichia coli, Klebsiella spp., Candida albicans, and Gram-negative bacteria, compared to the control group (P<0.01). In conclusion, diet additives increased egg yield, egg mass, and eggshell thickness, leading to better feed conversion rates due to their antimicrobial activity, compared to the control group.