A well-defined m-phenylenediimino-bridged ladder polymethylsiloxane (LP) was first synthesized through a well-defined ladder superstructure (LS) acting as synthetic template, which was self-assembled by concerted interaction of hydrogen bonding and aromatic π-π stacking of the monomer (M), N,N,-bis(phenyldichlorosilyl)-m-phenylenediamine. Some key characterization data of LP and, in particular, the extremely vulnerable LS with very unstable Si―Cl and Si―N groups were given. The molecular weights (Mn) of LS and LP are 5010 and 10480, corresponding to about 15 and 46 monomer units, respectively. To monitor the real in situ status of LS in solution the XRD measurements of special freeze-drying samples were performed, demonstrating two characteristic peaks of ladder structure. Most importantly, both LP and LS display exceedingly sharp resonance absorption peaks with a half peak width (△1/2) as small as 0.3 of MeSi(NH)O2/2 moieties in 29Si-NMR spectra. It is well-known that the higher the regularity of the ladder polysiloxane, the narrower the resonance peak of the siloxane moiety on the ladder backbone, and the smaller the half peak width△1/2. Therefore, the very small values of △1/2 for both LS and LP confirm that both LS and LP possess well-defined ladder structures.