For examination of a constraint release (CR) contribution to relaxation in monodisperse systems of moderately entangled six-arm star polyisoprenes (PI), viscoelastic measurements were conducted for blends of these star PI and a high molecular weight (M) linear PI. In the blends, the linear PI was dilute and entangled only with the matrix star chains. The terminal relaxation of this dilute linear probe occurred through competition of reptation and Rouse-type CR, as confirmed from its relaxation mode distribution. The probe relaxation time tau(probe) measured in the blends was utilized in the following way to elucidate the CR relaxation in the star matrices: Since the CR time tau(CR) of the star matrix is expressed as (2N(a))(2)tau(life) with 2N(a) and tau(life) being the entanglement number per two arms (span length) and the effective entanglement lifetime in the system, tau(CR) can be evaluated if the tau(life) value is known. For determination of the tau(life) value, the tau(probe) data of the linear probe in the star matrices was compared with the previously obtained tau(probe) data of the same probe in linear PI matrices under a molecular idea that tau(life) should be the same in a pair of star and linear matrices giving the same tau(probe) value. The molecular weight M-L,M-mat of the linear matrix paired with each star matrix was thus specified, and the value of tau(life) in the star matrix was determined by utilizing this M-L,M-mat value in a previously obtained empirical equation of tau(life) in the linear matrices ( tau(life) = 2.5 x 10(-18) M-L,mat(3) s at 40 degrees C). For the monodisperse systems of the star PI, the tau(CR) (=(2N(a))(2)tau(life)) thus evaluated was close to the measured relaxation time, indicating that the CR mechanism significantly contributes to the star relaxation. This result was in harmony with the validity of the molecular picture of partial dynamic-tube-dilation (p-DTD) confirmed for the star PI.