Strain wave gearing which is a compact mechanical device producing high mechanical advantages and speed reduction, has been used to accurately position the arms and wrists of robots. It is important to clarify the mechanism of the rotational transmission error of strain wave gearing. In this study, rotational transmission errors of cup-type strain wave gearing are measured over one revolution of an output shaft, and the frequency analysis is carried out for the measured results. Furthermore, effects of assembly errors and torque on the rotational transmission error are deter-mined experimentally. The main results are as follows. (1) The main frequency components of the rotational transmission error are even multiples of relative rotational frequencies of the circular spline, flexible spline and wave generator. (2) The frequency component of rotational transmission error, which is 2 cycles per wave generator rotation, is easily influenced by the inclination of the circular spline. (3) The frequency components of rotational transmission error which are 1 and 2 cycles per wave generator rotation are easily influenced by the offset between the circular spline and flexible spline. (4) The frequency component which is 13 cycles per wave generator rotation is observed when a torque is applied.