Poormirzaee, R.. Hamidzadeh Moghadam, R. and Zarean, A.. 2015. The application of PSO to joint inversion of microtremor Rayleigh waves dispersion curves and refraction traveltimes. Journal of Seismic Exploration, 24: 305-325. The accurate estimation of shear wave velocity (Vs) by Rayleigh wave dispersion analyses is very important for geotechnical and earthquake engineering studies, but dispersion curve inversion is challenging for most inversion methods due to its high nonlinearity and mix-determined trait. In order to overcome these problems, the current study proposes a joint inversion scheme based on a particle swarm optimization (PSO) algorithm. Seismic data considered for designing the objects were the Rayleigh wave dispersion curve and seismic refraction traveltime. For joint inversion, the objective functions were combined into a single function. The proposed algorithm was tested on two synthetic datasets and also on an experimental dataset. Synthetic models demonstrated that the joint inversion of Rayleigh wave and traveltime returned a more accurate estimation of Vs compared with single inversion Rayleigh wave dispersion curves. To verify the applicability of the proposed method, it was applied at a sample site in Tabriz city. northwestern Iran. For a real dataset. the refraction microtremor (ReMi) was used as a passive method for obtaining Rayleigh wave dispersion curves. Using PSO joint inversion, a three-layer subsurface mode] was delineated: the first layer’s velocity was 316 m/s and its thickness was 5.5 m, the second layer’s velocity was 280 m/s and its thickness was 2.8 m, and the last layer’s velocity was 512 m/s. The results of synthetic datasets and the field dataset showed that the proposed joint inversion technique significantly reduces the uncertainties of inverted models and improves the revelation of boundaries.