Dynamics of guest linear polymer trapped in permanently cross-linked network is investigated using the end linked poly(dimethylsiloxane) (PDMS) networks with well-defined mesh sizes (M-x) containing unattached linear PDMS. Viscoelastic relaxation behavior of the PDMS networks with the unattached PDMS has been examined as a function of the molecular mass of the guest PDMS (M-g) and the end-reactive precursor PDMS. Two host networks with different mesh characters and sizes, i.e., M-x approximate to M-e (trapped entanglement dominant) and M-x < M-e (cross-link dominant), were prepared by end-linking the sufficiently longer and shorter precursor PDMS relative to entanglement spacing (M-e), respectively. The dynamic mechanical measurements reveal that the networks containing the guest chains show the definite maxima in loss factor tan delta at certain frequencies which are attributed to the viscoelastic relaxation of the guest chains, and in addition, the location of the tan delta peak shifts to the lower frequencies with increasing in tau (g). The characteristic time tau (g), defined by the inverse of the frequency at the tan delta maximum, varies with M-g in good agreement with the predictions of the reptation theories, regardless of M-x. Interestingly, the reptation of the guest chains in the cross-link-dominant network of M-x < M-e is markedly retarded relative to that in the entanglement-dominant network of M-x approximate to M-e: tau (g) in the crosslink-dominant network is larger than that in the entanglement-dominant network by a factor of nearly 10(5). Such remarkable slowing-down of reptation is not simply explained in terms of only the reduction in M-x, which suggests that the effects of the high concentration of cross-link in the network of M-x < M-e on the dynamics of guest chains must be considered.