Methane emission fluctuations are strongly related to the barometric pressure changes. Variations of barometric pressure increase the occurrence of fires and explosions of underground mines. As part of the project supported by the Australian Government Coal Mining Abatement Technology Support Package (CMATSP), the methane concentration at tailgate return and the barometric pressure in the longwall panel were monitored and analysed. It was found from the monitoring that when the amplitude of barometric pressure change reaches +/ - 0.5%, the emission methane concentration measured changed from the minimum value of 1% to 1.5%. In order to understand better the mechanism behind the phenomena, a 3D computational fluid dynamics (CFD) model developed during the project is used to investigate the effect of barometric pressure on methane emission. Intensive studies on the effect of different parameters on the fluctuation amplitude of methane emission are conducted including the period of barometric pressure variations, the size of the longwall goaf. It is found that the longwall goaf formed during the longwall mining performs like an air accumulator in storing and releasing methane. Barometric pressure changes with smaller period causes larger changes of methane concentration at the return, and larger goaf in size causes larger variation amplitude of methane concentration. Concentrations of methane drawn from vertical boreholes are also affected by the barometric pressure changes.