In robotic assembly based on position control for assembly of small parts with small tolerances, even a slight position error would result in assembly failure. In this study, the assembly strategy for the mainboard and battery of a tablet PC is established for the assembly based on position control. And the assembly state estimation algorithm is proposed to estimate whether the assembly state is successful or not by using the pose and F/T sensor data. The performance of the proposed assembly state estimation algorithm was verified by experiments using a 6 DOF articulated manipulator.