We present a method of measuring the in situ superficial seafloor sound speed by using two receivers deployed vertically in sediment,based on the dispersion characteristic of normal modes in shallow water.Warping transformation is adopted to extract the first normalmode from the broadband propagation signals.Some experimental results which could validate the method are shown.One of the advantages of the method is that the seafloor sound speed can be solved directly from the relative vertical transmission loss of the first normal mode without the exact priori information of the environmental parameters such as the sound speed profile,the water-column depth and the source location.