Two new borates, Sr8MB18O36 (M = Zn or Cd) with expected formulas representing the Sr:M ratio of 8:1, were synthesized. Their structures were determined by single-crystal X-ray diffraction. They both crystallize in the same space group R (3) over barc and possess isolated [B18O36](18-) anionic group. In this work, not only Sr8MB18O36 (M = Zn or Cd) crystals were designed but they were also synthesized successfully. In addition, the reasons why the ratio of Sr:M is 8:1 in Sr8MB18O36 (M = Zn or Cd) are explained.