Temperature dependencies of the optical rotation and size exclusion chromatography with low-angle laser light scattering (SEC-LALLS) were performed on the sonicated K+, kappa-carrageenan in KCl salt solutions. The molecular weights were measured at the temperatures of the coil, helix, and coil-helix transition regions. The temperature at each region was determined by the optical rotation experiment. In a solution of low KCl salt concentration, the molecular weights did not change at any of the temperatures investigated. In this case, no hysteresis in the temperature dependence curve of optical rotation was observed. On the other hand, when the salt concentration increased, the molecular weight increased according to the transition from the coil to a helical conformation. In this case, a hysteresis was observed in the optical rotation curve. Moreover, the greater the increase in the KCl concentration, the greater the increase in molecular weight. These results indicate the existence of multiple conformational states for kappa-carrageenan that depend upon the solution conditions. That is, kappa-carrageenan makes a conformational transition from the coil to a single helical conformation at low KCl salt concentrations. However, kappa-carrageenan takes on a helical conformation with a dimerized molecular weight when the salt concentration increases. These dimerized helical molecules further aggregate when the salt concentration increases further.