The structures of the thin films of pentacene (C22H14) grown by organic molecular beam deposition on Au(111) have been investigated by scanning tunneling microscopy (STM). In the coverage range of 1-4 monolayers, pentacene molecules arrange into striped structure where each row consists of pentacene molecules aligned side-by-side. Based on the two dimensional (2D) unit cell as well as step height of the molecular island, we conclude that pentacene molecules form close-packed layers with their long molecular axis parallel to the surface. Although there is a possibility for the molecules to be tilted around their short molecular axis in this form, there is little difference between monolayer and multiplayer structure. Within the molecular layer and between adjacent layers, there is a broad distribution of relative angles in different domaims. The relative interaction strength between pentacene molecules in the same layer and in adjacent layers is discussed.