Single-molecule device studies have traditionally focused on highly conducting examples. However, molecular insulators are as important as molecular conductors for progress in nanoscale electronics. This work innovatively reveals the promising prospects and the superiority of siloxane molecular rings to achieve the same function as SiO₂ on the single molecule scale, functioning as a molecular insulator in electronics, and theoretically analyzes the size-dependent electron transport decay of siloxane ring-based single-molecule devices in contrast to alkane ring-based ones. Results indicate that siloxane ring-based devices possess stronger transport suppression and steeper transport recession than alkane ring-based ones, which is attributed to weaker electronic coupling through the backbone of siloxane rings and is intrinsic to the nature of the Si–O bond. Furthermore, the electron transport decay in each kind of molecular ring-based device originates from the increasing degree of overall structural fluctuations in the molecular rings.