The viscoelastic properties of 30 wt% solutions of styrene(S)‐butadiene(B) radial (star) block copolymers in dibutyl phthalate (DBP) have been measured to investigate the effect of temperature. Block structure used was(S—B—)mand(B—S—)n,wheremandnare about 5. The solvent, dibutyl phthalate (DBP), is a good solvent for the PS block and a poor solvent for the PB block. The temperature dependences of the storage modulusG′and loss modulusG″for the solutions exhibit a rheological transition atTc=66∼68°C.On raising the temperature,G′decreases remarkably at the temperatures and correspondinglyG″jumps up almost discontinuously. On lowering the temperature,G′andG″show the apparent hysteresis behavior caused by slow kinetics of phase separation and values ofG′andG″return to the original levels. When the temperature is lower than the transition temperatureTc,the measuredG′is higher thanG″,and whenT>Tc,G″>G′.These results suggest that the rheological transition atTcis associated with phase transition between a liquid with domain structures and a homogeneous solution. The rheological transition temperature coincides with the temperature at which differential scanning calorimetry (DSC) heating curves for the samples show an endothermic peak. According to the rheological behavior investigated in this study, it was found that the samples re‐form the original domain structure after 50 min at 60°C.