The objective was to measure the effect of temperature–humidity index (THI) on egg-laying and hatching parameters. Quails were randomly transferred to two cage blocks at 20.55 ± 0.05 °C and 52.25 ± 0.68% relative humidity (RH), accepted as 66 THI group. One cage block was transferred to the next room at 16.15 ± 0.13 °C and 70.77 ± 0.38% RH considered as 61 THI group. In the second stage of the study, the cage block previously accepted as 61 THI group was considered as to 69 THI group by rising up the temperature to 21.97 ± 0.18 °C. The group previously considered as 66 THI group was accepted as 76 THI group by rising up the temperature to 28.06 ± 0.02 °C. Feed consumption and egg weight were significantly decreased, in 69 and 76 THI groups. Daily egg production was lower in 61 and 66 THI groups than that in 69 THI group. In 76 THI group, shell weight and yolk height were decreased. High THI caused negative effects on feed consumption, egg weight, egg quality, and feed conversion ratio.