Dynamic mechanical properties of systems of a styrene‐butadiene‐styrene triblock copolymer (cSBS) inn‐tetradecane (C14) were examined as a function of concentration and temperature. Since the solvent C14 dissolves B blocks but precipitates S blocks, gels with quasinetwork structure composed of rigid S domains connected with B blocks are formed at room temperature. Such a gel exhibits rubberlike linear viscoelasticity under small‐amplitude oscillation. When the temperature is raised, the rubbery gel changes to a plastic fluid with viscoelasticity, and further to a viscoelastic liquid. Each of these transitions takes place within a relatively narrow temperature range. The critical temperature for the rubbery‐to‐plastic transition depends only slightly on the cSBS concentration, while that for the plastic‐to‐viscous transition depends strongly on the concentration. These results are interpreted in terms of the temperature and concentration dependences of the stability of the microdomain structure existing in the system.