Abstract: On the basis of joint observations of TC-1 at about -12.5R_e and Geotail at about -8 ～ -9R_e in the near-Earth plasma sheet in the magnetotail, the plasma instabilities in the vicinity of substorm onset occurred at about 02:53:20UT on 28 September 2004 were investigated. The observation results indicate that (1) the location of substorm onset triggering region is small within the inner plasma sheet in the near-Earth magnetotail, (2) the lower-hybrid instability is incited in the substorm triggering region and the lower-hybrid waves with the quasi-perpendicular propagation occur during substorm onset, (3) the magnetic dipolarization and the magnetic disturbance in the outside of near-Earth plasma sheet are very weak during substorm onset. The multiple magnetic dipolarization with different feature took place during substorm onset and substorm expansion phase. Key words Plasma sheet, Substorm, Magnetic dipolarization, Magnetic disturbance