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Palu City is a city on the island of Sulawesi with very complex geology. There are many active faults on Sulawesi Island, one of which is the Palu-Koro Fault system which extends NNW-SSE direction. Thick sediment under Palu City can cause amplification of the soil during an earthquake so that the damage will be quite severe. In order to characterize the subsurface structure of the City of Palu, microtremor data processing was carried out from 22 stations using the Horizontal-to-Vertical Spectral Ratio (HVSR) method. HVSR is a method for obtaining subsurface information from a single station measurement that can be used to examine seismic risk. HVSR is measured by comparing the Fourier spectrum of horizontal components with vertical components. The amplitude of this ratio, H/V curve, is varying with frequency. The H/V curve containing amplification information and the related dominant frequency which can then be inversted to obtain subsurface information, such as S-wave velocity and layer thickness. In this study, the inversion was carried out by using the PSO (Particle Swarm Optimization) algorithm. The PSO algorithm is stochastic optimization technique inspired by social behavior of bird flocking when searching a promising area for food. In order to evaluate the consistence of obtained results we also compare with the model from OpenHVSR program. The obtained shear velocity model shows that low velocity zone (less than 700 m/s) related to the sediment layer is around 200 m with a relatively small misfit at around 1.3.