The properties of the inner core boundary (ICB) carry important information about the growth of the solid inner core and solidification of the liquid outer core. In this presentation, we study the ICB by analyzing the spectrum difference of PKiKP (a compressional wave reflected from the ICB) and PcP (a compressional wave reflected from the core-mantle boundary) phase pair. Our dataset includes the seismic data recorded by the Hi-net in Japan for two earthquakes occurring in Myanmar on 4th September 2009 and 4th February 2011, with their PKiKP waves sampling the ICB beneath mid-eastern China. Little waveform and spectrum differences are observed between PKiKP and PcP waves in the seismic data. By comparing spectrum shapes of the seismic data and the synthetics of models with various topographic fluctuations and transitional thicknesses of ICB, we conclude that the studied ICB region is flat and sharp. Within the resolvability of the seismic data, the ICB topographic fluctuation of the region is less than 200 m in height and the transition of the boundary is thinner than 100 m in thickness.