Kriging method is a frequently used interpolation methodology in geography, which enables estimations of unknown values at certain places with the considerations of distances among locations. When it is used in transportation field, network distance is a better measurement of distance as traffic follows the network. This report presents the development of the Network Kriging method and demonstrates its application on predicting transit ridership. Network distance, instead of Euclidean distance, is used to reflect the fact that subway stations are only connected by subway tunnels. Results show that the Network Kriging method outperforms other approaches. And the application on transit ridership estimation indicates that the new service would largely relieve the traffic burden on current crowded subway lines, although the total fare revenue would not increase right after the new service.