Gripping and holding objects are key tasks for robotic manipulators. The development of universal fingers able to pick up unfamiliar objects of widely varying shapes and surfaces is a very challenging task. Passively compliant underactuated mechanisms are one way to obtain the finger which could accommodate to any irregular and sensitive grasping object. The aim of the underactuation is to use the power of one actuator to drive the open and close motion of the finger. The underactuation can morph shapes of the finger to accommodate to different objects. As a result, the underactuated fingers require less complex control algorithms. The fully compliant mechanism has multiple degrees of freedom and can be considered as an underactuated mechanism. This paper presents a new design of the adaptive underactuated compliant finger designed by topology optimization. The main points of this paper are in explanation of kinetostatic analysis of the proposed finger structure using approximate rigid-body model with added concentrated compliance in every single joint of the finger. The results can be used as estimation for gripping force or finger displacement.