The temporal variation of greenhouse gas (GHG) emission in a petrochemical wastewater treatment plant (WWTP) was investigated in this study. Two approaches including an in-situ continuous monitoring and a typical grab sampling methods were also compared. The in-situ continuous monitoring method provided more detailed information regarding the temporal variations of GHG concentrations. A sufficient sampling frequency (e.g., once every 6 hours) for the grab sampling method is required to effectively resolve the diurnal variations of GHG concentrations. This study highlights significant diurnal variations of GHG concentrations in different wastewater treatment units. Only with proper and reliable sampling and analytical methods, it becomes possible to correctly identify the characteristics of GHG emissions and to develop strategies to curtail the GHG emissions from such an important source in response to regulatory measures and international treaties. This study revealed that N2O was the dominant species responsible for GHG emissions in the WWTP and the emission factors of CH4 and N2O were higher in the equalization tank and final sedimentation tank compared to other units. We further compared the GHG emission factors of this study with other literatures, showing that the GHG emission factors were lower than those measured in Netherlands, Australia, and IPCC, but similar to those measured in Japan.