The modulated structure of Bi2Sr2CaCu2Oy single crystals has been observed and studied in detail with an X-ray rotating-anode diffractometer, indicating that the satellites of Bragg reflections are stretched along the c* direction in reciprocial space. It is the result of the stretch that the widened peaks are incompatible with a B-centered orthorhombic bravais lattice , and the intensities of satellites become asymmetry. The superlattice based on the lattice mismatch is not orthorhombic but monoclinic and perhaps forms superlattice twin structure in Bi-2212 single crystals.