The notch size effect of forged Ti-6Al-4V plate, loaded under high cycle fatigue (HCF) conditions, is investigated for both cylindrical and flat fatigue specimens. Cylindrical specimens were machined with three sizes of circumferential V-notches, with the smallest size dictated by the ability to machine within given tolerances. Flat dogbone specimens were machined with two sizes of opposite V-notches. All notches in both cylindrical and flat specimens have an elastic stress concentration factor of approximately Kt = 2.7. The state of stress at the notch root in the cylindrical specimens is biaxial. Thin flat specimens were chosen with the intent of minimizing the out-of-plane (through the thickness) stress component at the notch root. All specimens were cycled to obtain points on a Haigh diagram, also referred to as the modified Goodman diagram, for a constant life of 106 fatigue cycles. Specimens were cycled at stress ratios of R = 0.1, 0.5, and 0.8, at a frequency of 50 Hz. A step loading technique for generating a point on the Haigh diagram from a single test specimen was used, where the fatigue limit is found by fatiguing the specimen for 106 cycles at a stress level below which failure is expected. If failure does not occur, the stress level is raised and the specimen again fatigued for 106 cycles. This procedure is repeated until failure, after which the fatigue limit is calculated by interpolation. The validity of this technique for Ti-6Al-4V was demonstrated by generating constant amplitude stress-life data for comparison with the interpolated fatigue limits. The notch size effect is examined in light of the stress state at the notch root, and the appropriateness of the von Mises effective stress in characterizing the multiaxial state of stress in HCF is considered.