Abstract: Convergent beam electron diffraction （CBED）, combined by computer simulation, has been to be a powerful tool for the strain study, due to its high spatial resolution and high sensitivity. The local lattice parameters and the strain in the graded Ge_xSi_1-x grown on Si buffer by molecular beam epitaxy（MBE） have been determined by the technique. The results indicate that lattice parameters and the strain in graded Ge_xSi_1-x increase while Ge content, x value, increase from 0.05 to 0.35.The lattice of the whole graded layer is tetra-symmetry basically although some slight deformations have been observed from CBED) pattern around the interface between graded GeSi and the Si buffer.There are thread dislocations with density as high as 10⁷/cm² in the middle area of the graded layer, but fewer exist in the top, which is attributed to the distribution and relaxation of the strain in the material system.