Waste potato starch (WPS) was obtained from the food industry during slicing, peeling and washing potatoes in the process of producing potato chips. After it was purified with H 2 O 2 and NaOH at the desired temperature, carboxymethyl potato starch (CMPS), with the DS in the range from 0.04 to 1.24, was synthesized in isopropanol (IPA) and NaOH solution by adding monochloroacetic acid (MCA). Considering all the studied conditions, the best results were achieved when IPA was used as solvent at 45 °C for 180 min and at a NaOH/MCA molar ratio of 2.2:1. CMPS-2, obtained under these conditions, reached a viscosity of 14200 mPa. s (for 2% CMPS in water at 1 rpm) for the highest DS. The samples were investigated by determining their DS and by SEM, FTIR, XRD, NMR and thermal analyses. Moreover, the viscosity of the CMPS aqueous solutions was measured. 100% cotton yarns with different numbers (10/1, 20/1 and 30/1 Ne) were sized with CMPS-29, CMPS-27, CMPS-25 and CMPS-22, and then their tensile strength and elongation at break were measured. The tensile strength of the sized samples increased by 87.65%, compared with that sized with WPS. Also, CMPS with DS 0.04 exhibited considerably better sizing properties, compared to WPS.