Abstract: In this paper, the optimal grid size is studied based on link model reachable with probability in realistic environment. Reaching probabilities of neighboring grids and path are analyzed, and the effects of node density and grid size on reaching probabilities is studied, and optimal grid size of which maximize energy efficiency and network average packet delivery ratio is analyzed theoretically. A scheme of combination of packet delivery ratio and energy efficiency to select grid size is proposed, which maximizes network lifetime when meet users requirements. The grid routing is simulated and analyzed, the results show the optimal grid size of theoretical value and simulation value to be equal almost, and the grid size selecting scheme of combining packet delivery ratio and energy efficiency can prolong network lifetime efficiently.