The upper mantle electrical conductivity structure of the pacific back-arc beneath northeastern China was investigated through long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) experiments. MT and GDS responses were obtained up to periods of 105~106 seconds, and were inverted to one-dimensional (1-D) models with minimum and smooth structure constraints, respectively. The resulting conductivity model was compared with past models and the mantle transition zone beneath northeastern China is more conductive than those of other tectonic settings by almost one order of magnitude in the depth range of 400~600 km. This feature may correspond to the presence of a stagnant slab beneath this region. In the mantle at a depth of less than 400 km, the conductivity profile has a similar feature to that in the thermal and extensional tectonic region in the southwestern United States. ¿ 2001 American Geophysical Union