Many underactuated grippers with fingers have been developed these last years. Their drawback is that they only ensure conditionally grasp stability. This paper presents a study of the grasp stability of an isotropic underactuated finger, which is made by two phalanxes and uses cams and tendon for actuation. The paper presents also a study of the internal forces developed in the transmission chains. The proposed model serves for the gripper using as for part dimensioning.