The purpose of this investigation is to find a suitable molecular structure for stabilizing additives for coal-water mixtures (CWM). Rheological characteristics and stability of CWMs with the addition of three kinds of additives (bio-polysacharides; S-60, S-130 and S-194) were measured. Although these additives have almost the same length of molecular chains, they have different size of ramified chains (branches). The experimental results showed that S-194, which has long branches, and S-60, which has no branches, were both effective in increasing stability. In the case of S-194, long branches may contribute to the buildup of network structures in CWM. On the other hand, S-60 was found to show high viscoelasticity with the addition of small amounts of ionic species and these characteristics were effective to increase the stability of CWM. It is concluded that the stabilizing mechanisms of S-60 and S-194 were different.