One of the important features to understand the dynamics of humanoid robots is the absence of the link fixed in the inertia frame. When the robot has no external reaction force through the interaction with the environment, the generalized force which actuates 6 DOFs of the base-link of the robot is always zero. This constraint is related to the conservation of angular momentum which means a nonholonomic constraint, thus the motion generated by the dynamics constraint contains properties of the dynamics information of the system. In this paper, we propose an identification method based on the floating-base dynamics, when the system has no external force. Using this method, inertial parameters can be identified without force measurement, only from motion data. This paper also shows the identifiability of the proposed method for floating open kinematic chain which can actuate all the joints and change the initial values of linear and angular momentum. The method has been tested on two examples; a simple chain consisted of two links and the human body dynamics.