A method of diagnosing failure of fuel level sensors includes a first measuring step of measuring voltages from first and second failure detection terminals; if the voltage is higher than a reference, diagnosing the voltage as “high,” and if the voltage is lower than a reference, diagnosing the voltage as “low;” and if the voltage is neither high nor low: a second measuring step of receiving the voltages from the first and second failure detection terminals; calculating and storing a difference between the voltages of the first and second failure detection terminals; determining whether an amount of fuel is reduced; if the amount of fuel is reduced, calculating a difference between voltage signals of the first and second failure detection terminals; determining whether the voltage difference before and after the amount of fuel was reduced are identical; and if the voltage differences are identical, returning to the first measuring step.