Thermal-induced gelation of polyacrylonitrile (PAN)/dimethyl sulfoxide (DMSO) concentrated solution has been theologically investigated in relation to temperature and aging. Phase behavior of PAN/DMSO solution was studied through examining the intrinsic viscosities [eta]. The gelation temperature T-g, gel time t(g), and the critical values of relaxation exponent n were obtained by measuring the viscoelastic functions (G', G '', tan delta, and eta*) based on the Winter and Chambon theory. The solution with higher PAN concentration was found to be more sensitive to the temperature, which makes it gel at a higher temperature. The n value, however, changes little with varied concentration. The gel strength S was found to depend on the concentration of the solution. Gelation of the 23 wt % PAN/DMSO solution occurs after being aged at 25 degrees C for a period of time, i.e., gel time t(g). The critical n value (S value) at t(g) was larger (smaller) than that at T-g in the cooling process, attributed to the different critical feature of the PAN gels formed in different conditions. The power law relations eta(0) alpha epsilon(-gamma) and G(e) alpha epsilon(z) were found to be valid for PAN/DMSO solution before and beyond the gel time t(g), respectively.