The rhombic distortion of the CuO 4 basal plane in the tetragonal-low-temperature (TLT) phase (P4 2/ncm) of La 1.6 − xNdo .4Sr xCuO 4 has been investigated from high-resolution powder neutron diffraction at 10 K. It has been found that the rhombic distortion reaches a maximum at x = 0.115 in the TLT phase of 0.10 ≦ x ≦ 0.25. This correlates very much with the result that the local minimum of Tc as a function of x appears at x = 0.115. It is concluded that the suppression of superconductivity in the TLT phase depends not on the structural-phase-transition-temperature T d2 to the TLT phase but on the magnitude of the rhombic distortion of the CuO 4 basal plane.