The thickness of upper mantle discontinuities (at depths around 410 km and 660 km) provides a critical clue to our understanding of deep earth. Since the amplitude of reflected or S-P converted waves is controlled by the thickness of discontinuities, short-period studies of such waves may give a fine constraint on the thickness of upper mantle discontinuities. In this study, we analysed short-period and vertical waveform data from 14 deep events in the Fiji-Tonga region. The data were obtained from J-Array, a short-period and large-aperture seismic array in Japan. With slant stack analyses of these data, we detected short-period reflections or S-to-P conversions from 410 km and 660 km discontinuities near sources, which implies that both discontinuities are quite sharp. To directly measure the frequency content of these waves, linear slant stacks were made with various frequency pass-bands (0.2--0.5, 0.5--1.0 and 1.0--2.0 Hz). Synthetic traces were generated for the models with various thickness of the 410 km and the 660 km discontinuities. Comparison of slant stacks with synthetic traces indicates that the thickness of both discontinuities is at most 5 km. ¿ American Geophysical Union 1994