Abstract: By comparison with original sweet potato starch, the physico-chemical characteristics of sodium carboxymethyl sweet potato starch ( CMS-Na) with different degrees of substitution were studied in this paper. Studies and analysis included the followings:the viscosity determination, viscosity curves for different temperature and pH, and with the existence of different concentration of electrolyte ( NaCl) and non-electrolyte ( sucrose) , the freezing stability, the grain shape and X-ray analysis. The results indicated that the viscosity of CMS-Na was higher than original sweet potato starch, the existence of electrolyte ( NaCl) and non-electrolyte ( sucrose) influenced the viscosity of CMS-Na greatly, the gel intensity of CMS-Na was weak and great changes had taken place in the shape and crystal structure of CMS-Na.