Supermolecular structures for poly(dimethylsiloxane) (PDMS) gels with unimodal and bimodal distributions of the chain lengths between the crosslinks are investigated by a small angle X-ray scattering (SAXS) technique. The unimodal and bimodal networks are prepared by the end-linking reaction of oligomeric and polymeric dimethylsiloxane mixtures with a series of compositions, and they are fully swollen in toluene. The scattering profiles for unimodal networks primarily depend on whether the precursor chains are highly entangled or not in the uncrosslinked state. For bimodal networks, the correlation lengths Z for the supermolecular structures do not strongly depend on the mixing ratio of the precursor chains, and the values are close to " for the unimodal network of oligomeric chains. The value of % for the unimodal network of polymeric chains is larger than for the unimodal network of oligomeric chains, while it is considerably reduced by adding a small amount of the oligomeric chain. The experimental results are interpreted on the basis of the shortest closed circuits concept describing spatial nonuniformities formed by end-linking, and the consideration that the elementary meshes in the networks composed of long precursor chains are mainly formed by entanglements.