This paper presents the theory of passive localization for underwater sources based on multipath arrival structures. The principles of channel modelling in Ray-theory, determination of eigenrays which connect source and receiver, the information of source range and source depth is included primarily in arrival angles and time delays. The weighted-subspace-fitting (WSF) technique is used to minimize the distance between the signal subspace and the space spanned in a finite range-depth space. The information of arrival angles is contained in the signal subspace and thus this technique does not estimate the arrival angles. However, this method is only effective in the estimation of source range. The multipath arrival time delays are sensitive to the source depth, so a method based on the time delays between multipath arrivals is presented to determine the source depth. By combining the above two methods, the source location can be estimated more accurately.