S-wave propagation from source to a high impedence contrast boundary can produce convert phase SP. Normally S-phases are recorded on radial component and SP-phase on vertical component. Using three component data, these two phases can be identified and isolated. Some significant structure information can be studied from these two phases. The Q value of sedimentary rock is one of them. The spectral ratio of SP- and S-phases are calculated in this study for estimating frequency-independent Q value of near surface layer. By the help of P- and S-waves velocity structure, the travel times of SP- And S-phases can be estimated such that SP-phase is identified. Three SP-phases, converted from different layers in the Lotung area, can be identified. The earliest SP-phase is possibly converted from the bottom of PLEISTOCENE formation. The second SP-phase might be converted from the bottom of ALLUVIUM layer, and the last one might be converted from the bottom of surface soil layer. Each SP/S ratio gives a different Q value which shows the variation of Q in each layer. The range of Q value estimated by this method agreed with the laboratory measurement of sedimentary rock.