Partially gelatinized corn starch (PGCS) was used for sizing cotton yarn in the search for potential sustainable and environmentally friendly warp-sizing agents as substitutes for poly vinyl alcohol. PGCS was prepared by a constant temperature method (CTM). The viscosity stability of PGCS was investigated and characterized by wide-angle X-ray diffraction, differential scanning calorimetry, polarized light microscopy and laser diffraction particle size analyzer. The results indicated that the viscosity of CTM65 (starch milk held in a water bath shaker at 65 °C for 30 min) was stable. The cotton yarn was sized by PGCS (CTM65) on a cylinder slasher sizing machine. Scanning electron microscopy indicated that starch granules on the surface of the yarn had completely gelatinized and formed a film when the yarn was treated via the No. 2 drying cylinder (to reduce yarn hairiness). This research supports the idea that PGCS could improve the viscosity stability of native starch paste and have potential as an environmentally friendly warp-sizing agent.