Granular carboxymethyl starch (CMS) with high degree of substitution (DS) was synthesized using a modified dry process. The carboxymethylation reaction was carried out with granular potato starch, solid NaOH and sodium monochloroacetate (SMCA). The effects of reaction temperature, reaction time, molar ratio of NaOH and SMCA to starch on the degree of substitution (DS) and the reaction efficiency (RE) of CMS were investigated. High DS up to 0.78 was obtained for reaction at 323 K for 6 h with molar ratio of NaOH and SMCA to starch being 2:2:1. CMS was characterized by Fourier transform infrared spectrophotometer and scanning electron microscopy.