The stress relaxation of ultrahigh molecular weight polyethylene (UHMWPE) gels in a broad composition range (0.025 < phi(P) < 1.0) was studied within the framework of the reptation model. The molecular weight distribution (MWD) was determined at an optimum gel concentration (phi(P) = 0.365) by regularization analysis of the stress relaxation data. The calculated relaxation moduli [G(t)] by using MWD evaluated at a certain composition (UHMWPE gels, phi(P) = 0: 365) were consistent with the experimental G(t) of other compositions in a broad range (0: 218 <= phi(P) <= 1.0). That is, the MWD (regularization method) was self-consistent within the framework of the reptation model. The composition dependence of the monomeric friction factor (zeta(eff)) of the UHMWPE gels was examined by use of the double reptation (DR) model with the determined MWD. The Ferry's free volume model of the molecular weight dependence of zeta(eff) can be extended to the composition dependence of zeta(eff). In the low-concentration gels (0: 025 <= phi(P) <= 0: 17), the experimental Gd(t) relaxed faster than Gd(t) expected by the DR model. (C) 2015 The Society of Rheology.