Abstract: With the progress of urbanization, urban expansion has exacerbated the contradiction between human beings and natural ecosystems. Thus, optimizing the urban spatial pattern has become an important measure for the sustainable urban development. Based on DMSP-OLS and NPP-VIIRS nighttime lighting data, the urban built-up areas of the core area of Guangdong-Hong Kong-Macao Greater Bay Area (GBA) from 1998 to 2017 were extracted. Then the GIS spatial analysis methods and a series of urban expansion indicators were employed to reveal the spatial and temporal changes of built-up areas of the core area of GBA and their driving mechanisms were quantitatively analyzed. The results show that urban built-up areas in the core area of GBA generally showed an upward trend while the rate of expansion showed a significant downward trend. Overall, the closeness of the urban built-up area in the core area of GBA decreased with time and the fractal dimension basically remained unchanged despite the fluctuations. During 1998-2017, the expansion of the urban built-up area in the core area of GBA spatially showed a "northwest-southeast" direction. Meanwhile, the center gradually moved towards the northwest. In addition, the urban expansion pattern presented a diffusion-condensation-diffusion oscillation. Physical factors and socio-economic factors had obvious correlation with the expansion of urban built-up areas while their influences varied within different periods.