To develop a molecular imaging probe for potassium cation based on the chemical shift change of F-19 magnetic resonance (MR), thrombin aptamer, whose structure was changed with the selective binding of potassium cation, was labeled with a fluorine compound. We demonstrated that the probe could detect potassium cation by chemical shift change of the F-19 MR signal, and that the chemical shift change was caused by potassium cation selective G-quadruplex formation.