This study investigated the effects of water temperature and photoperiod on oxygen consumption (OC) in the giant grouper Epinephelus lanceolatus. OC rate in the giant grouper at 17, 22, 27 and $32^{circ}C$ were $61.7{pm}0.4$, $72.2{pm}0.6$, $102.9{pm}0.8$ and $141.7{pm}1.0mg;O_2/kg/h$, respectively, indicating a linear increase in OC with water temperature. Photoperiod was regulated in accordance with the light (06:00~18:00 h, L) and dark (18:00~06:00 h, D) phases of the diel cycle (12L/12D), with a water temperature of at 17, 22, 27 and $32^{circ}C$. OC rates during the light and dark phases were $62.7{pm}0.4$, $62.5{pm}0.3mg;O_2/kg/h$, respectively, at $17^{circ}C$. No significant differences were observed between the light and dark phases (P>0.05). OC raters during the light and dark phases were $74.8{pm}0.7$, $69.6{pm}0.6mg;O_2/kg/h$ at $22^{circ}C$, $107{pm}1.2$, $98.0{pm}0.7mg;O_2/kg/h$ at $27^{circ}C$ and $147.6{pm}1.1$, $135.8{pm}0.8mg;O_2/kg/h$ at $32^{circ}C$, respectively, indicating that OC is higher during daylight than nighttime. Giant grouper is thought to be a diurnal fish species, because the level of oxygen consumption during light period was higher than that during dark period.