Abstract: Internal force analysis for objects grasped by multiple robotic arms is presented in this paper.It is pointed out that the existing definition of internal force is not invariant to Euclid collineration. This leads to some difference between the modeled internal force and the real one. Load distribution and coordination control based on the wrong definition may cause damage to robotic arms and/or manipulated objects.With the use of two new projection operators,the forces imparted to the object by robotic arms are decomposed into two parts,one affects only the motion of the object and another leads to no motion of the object but internal force.Internal force defined in this way is invariant to Euclid collineration.