In this paper, the effects of a bearing ball of the wave generator and three kinds of assembly error on the real root stresses and the bending stresses at the cylinder and diaphragm parts of a cup-type flexible spline in a strain wave gearing (commonly known as a harmonic drive gearing) are analyzed. The following results are obtained. (1) Effect of the relative angular position between the major axis of the wave generator and the bearing ball on the real root stresses at the center part of the facewidth is large, and should be considered when the bending strength is evaluated, (2) Effect of the relative axial position between the wave generator and the flexible spline on the real root stresses of the flexbile spline is small. (3) Position of the maximum real root stresses of the flexible spline is at the center part of the facewidth when there is eccentricity or inclination in the circular spline assembly. (4) Eccentricity and inclination of circular spline assembly have little effect on the real root stresses at the tooth part, but have a considerable effect on the bending stresses at the cylinder part near the teeth and at the diaphragm part near the boss.