Stress relaxation in uniaxial extension has been studied in a linear and a four‐arm star‐branched polybutadiene and mixtures containing 10 and 12% of the former. Their weight‐average molecular weights(Mw)were 344,000 and 655,000, respectively, andMw/Mnwas approximately 2. Relaxation in the pure star was much slower than in the pure linear, and the behavior of the mixtures was intermediate in character. Relaxation of the linear polymer in the terminal zone at different stretch ratios λ could be described by the equationf(t)=E(t)(A0/λ)ln λ,wheref(t)is the time‐dependent force andA0the unstrained cross‐section area, andE(t)can be identified with the small‐strain relaxation modulus of linear viscoelasticity. The time dependence ofE(t)agreed rather well with the Doi‐Edwards theory with molecular weight distribution taken into account. For the mixtures, the contribution of the linear speciesE1(t)was calculated by difference from the equationE1(t)=E(t)−V2E22(t),whereE22(t)is the relaxation modulus of the pure star andV2is an empirical weighting factor obtained from comparison ofE(t)andE22(t)at long times. It was found that(1−v22)−1E1(t)agreed withE11(t)within experimental error. It appears that in a star‐rich mixture the linear species relaxes at the same rate as in its homologous environment.