In this study, the optimal design of bio-inspired elastomer microfibrillar structures with mushroom-shaped tip endings for maximal adhesion is investigated. Previous works have suggested that a mushroom-shaped microfiber could provide the highest adhesion among many other tip patterns. The optimal shape of the mushroom tip endings is characterized theoretically in terms of stalk diameter (at/a) and the tip ending wedge angle ($\theta$), and verified experimentally with precisely fabricated elastomeric microfibers.