Abstarct: Soil liquefaction is a phenomenon in which the shear strength of granular cohesionless soils will reduce or even vanish in a very short time under effect of dynamic loads such as earthquake or blast and the soil changes from a solid to a liquefied state. Detection of liquefaction occurance is one of the most important factors when a structure is constructed on a ground which is suspected to soil liquefaction or subjected to intensive dynamic loads such as earthquakes. There fore, it is needed to analyze earthquake waves during an earthquake happening. The most appropriate method for analyzing earthquake waves is the time- frequency analysis. In this thesis, the recorded accelerations of earthquakes in many liquefied and non- liquefied sites were analyzed by wavelet transfer using MATLAB® program and three-dimensional wavelet spectrum of each earthquake was determined. An index was defined and assigned which demonstrates whether the site is liquefied or non-liquefied. This index names Index of Liquefaction Detection (I.L.D.), shows the rational volume oscillations of 3D earthquake spectrum before and after PGA occurance and can be reached by the estimation of volume under earthquake 3D spectrum which determines ranges for both liquefied and non-liquefied sites and indicates that it is less than one for non-liquefied sites whereas it is more than one for liquefied sites.