Abstract-A frame of modular design problems and requirements for underactuated mechanisms is discussed as related to robotic fingers. The proposed modular mechanism is connected sequentially by series units of underactuated mechanisms, which have the feature of passive self-adaptive in grasp operation and uniformizable in design procedure. The design considerations for modular underactuated mechanism are outlined. Optimality criteria are analyzed with the aim to formulate a general design algorithm. An example of a four-phalanx modular robotic finger is studied as an improvement of new version LARM Hand with the aim to show the practical feasibility for the proposed modular concepts and design methods