This paper uses motion capture data to describe the interpolation of the upper body motion of humanoid robots. For the motion interpolation of humanoid robots, transition trajectory must be created. This trajectory should be based on consideration of the continuity between the first and second motions and the characteristics of humanoid robots, such as the limitations of the joint angle, velocity and torque. In this paper, we use a quintic polynomial to naturally interpolate two motions and we explore techniques of finding the optimal trajectory for humanoids robots. In particular, we find appropriate constraints of the polynomial at the second motion, thereby avoiding the mechanical limitations of the robot. These constraints can make the trajectory more reasonable. Finally, we successfully interpolated two motions and let the robot attempt various motions with continuity.