Bangkok, the capital city of Thailand, is located at a remote distance from seismic sources. However, it has a substantial risk from these distant earthquakes due to the ability of the underlying soft clay to amplify ground motions. Seismic microzonation of big cities, like Bangkok, provides a basis for site- specific hazard analysis, which can assist in systematic earthquake mitigation programs. In this study, a seismic microzonation map for the greater Bangkok area was constructed using microtremor observations. Microtremor observations were carried out at more than 150 sites in the greater Bangkok area. The predominant periods of the ground were determined from the horizontal to vertical (H/V) spectral ratio technique. A microzonation map was then developed for the greater Bangkok area based on the observations. The study shows that there is a great possibility of long-period ground vibration in Bangkok, especially in the areas near the Gulf of Thailand. This may have severe effects on long- period structures, such as high-rise buildings and long-span bridges.