Potato starch phosphate (PSP) particles were prepared via an esterification process of potato starch with a mixture of urea and three different concentrations of phosphoric acid, in order to introduce polar groups into the potato starch. These particles were then adopted as a potential anhydrous electrorheological (ER) material under an applied electric field. The PSP particle-based ER fluid exhibited typical ER behavior, with a yield stress under an applied electric field. We also constructed their universal yield stress, using a critical electric field in the potato starch-based anhydrous ER system, mainly focusing on the degree of phosphate substitution in the potato starch particles.