Abstract: This paper discribes the results of preliminary study on the structures of earth's crust and upper mantle surrounding the Dayawan gulf inferred from converted waves. Three observed profiles have been completed, with a distance of about 5—10km between observed points by use of 3-component short period seismograph, type DD-1 made in China. During about two months 60 distant earthquakes have been recorded to provide a number of available data for analysis of P to SV converted waves.Two stable groups of PS waves can be recognized in this region: PS_C and PS_M from the boundary of earth's crust. 1-2 groups of PS converted waves from upper mantle have been picked up as well, although they were shown unstable.Three cross sections of deep-seated structures have been made up along profiles. The Moho discontinuity and the boundary C in the crust can be determined and average depth of boundary C and M ranges from 12 to 14 km and 28 to 30km, respectively.Based on above-mentioned seismic cross sections we can outline the isobathy line map of boundary C and M for this region. On the map in question , evident is major feature of the local uplift in the central of the region, with a trend of NE and an amplitude of 2-3km. In correspondence with the Moho uplift, there is a local depression in isobathy line map of boundary C.The following conclusions can be drawn:1. Method of PS converted waves can be used efficiently for investigating the deep-seated structures in South China as well as in North China;2. The deep-seated structures in seismically passive zones of the Dayawan gulf are very different from those of the active zones in North China. The deep-seated structures in the region of the Dayawan gulf are characterized by smooth distribution of deep-seated boundaries, less change of layer thickness and a few faults on the boundary.