A study was conducted to investigate the seismic hazard microzonation of the central plain of Thailand, which is situated in an area with a thick quaternary basin. Unconsolidated sediments can lead to amplification of earthquake ground shaking at fundamental frequency and can cause a significant increase in damage to buildings. The research yielded significant results, including the development of a fundamental frequency map through the analysis of HVSR for 149 microtremor measurement sites. Subsequently, a Vs30 map was derived utilizing HVSR inversion techniques, and a soil classification map was constructed based on the NEHRP classification. The upper central plain along the Yom River and the Nan River had a low fundamental frequency of 0.3-0.5 Hz and low Vs30, which can be classified as soil type Class E. In the southern areas of Ayutthaya, Pathum Thani, and central Bangkok, an extremely low Vs30 of less than 100 m/s was observed, indicating soil class F or special soft soil. A comprehensive investigation was conducted on probabilistic seismic hazard analysis by considering the Vs30 sites condition for PGA, SA0.2s, and SA1.0s with a 2475-year return period. The northern region of the upper central plain and the western side of the central basin exhibit relatively high seismic hazards. Furthermore, the site effect significantly amplifies ground motion at the 1.0 second period, surpassing the earthquake-resistant design standard for buildings in Thailand by more than 5 times, particularly in the central region of the lower central plain.