Glucose reacts with proteins non-enzymatically under physiological conditions. Such glycation is exacerbated in diabetic patients with high level of blood sugar, and induces various complications. Human albumin serum (HSA) is the most abundant protein in plasma and is glycated by glucose. The glycation sites on HSA remain controversial among different studies. Here, we report two protein crystal structures of HSA in complex with either glucose or fructose. These crystal structures reveal that the presence of linear forms of sugar for both monosaccharides. The linear form of glucose forms a covalent bond to Lys195 of HSA, but not the case for fructose. Based on these structures, we propose a mechanism for glucose ring opening involving both residues Lys195 and Lys199. These results provide mechanistic insights to understand the glucose ring opening reaction and the glycation of proteins by monosaccharides.