This article presents an online remaining fatigue life estimation (prognosis) technique using nonlinear ultrasonic modulation. When two ultrasonic inputs at two distinct frequencies are introduced to a structure with a fatigue crack, modulation components at the sum and difference of input frequencies are generated owing to crack opening and closing. First, a fatigue index is defined for prognosis and extracted from nonlinear ultrasonic modulation measurement. Subsequently, it is shown that the fatigue index is expressed as a simple power function of loading cycles applied to a target structure based on the Nazarov-Sutin and Paris-Erdogan theories. Then, the real-time remaining fatigue life estimation method is proposed by fitting the power function to online data obtained from the target structure. The performance of the proposed prognosis technique is evaluated using either notched or welded aluminum (6061-T6) plate specimens.