Northeast India is bounded by the Himalayan arc to the north and the Burmese arc to the east. The Shillong Plateau and Assam Valley lie at the boundary zone of the two arcs. Three microearthquake surveys were conducted in this area from 1984 to 1986. We have applied a tomographic method to about 2800 high-quality P- and S-wave arrival times from 364 local earthquakes that were recorded in the magnitude range 2.0 to 4.0 by 22 temporary seismic stations during the surveys to determine the 3D velocity structure of the crust and upper mantle in this region. The result reveals significant lateral heterogeneities in the study area. The tomographic images obtained in this study are compatible with the major tectonic features such as active faults and seismicity trends.