Fatigue tests under plane bending and pulsating tension were carried out the notched FRP plates for a wide range of notch-root radii and stress amplitudes. An attention was focussed on the fatigue damage development near the notch root of specimens. The luminance at the limited spot near the notch root was measured successively during fatigue tests to evaluate the fatigue damage. Closer observation by means of a scanning electron microscope revealed that the initiation of microcracks at the notch root was accompanied with a decrease in luminance near the notch root. The experiment shows that the number of cycles to fatigue damage initiation is determined by both the maximum elastic stress at the notch root and the notch-root radius. On the basis of the concept of linear notch mechanics, the experimental results can be clearly elucidated and a criterion of fatigue damage initiation is determined in terms of a combination of the maximum elastic stress, σmax, notch-root radius ρ and the number of cycles to fatigue damage initiation Nd. The criterion is expressed as: σmax·(Nd)m=C(ρ), where m is the material constant. The parameter C is the material constant, which is governed by the notch-root radius only and is independent of other notch geometries and specimen size. By applying the criterion derived here, it is possible to make an accurate estimate of the fatigue life for notched FRP plates.