Natural surfaces with super hydrophobic properties often have micro or hierarchical structures. In this paper, the wetting behaviours of a single droplet on biomimetic micro structured surfaces with different roughness parameters are investigated. A theoretical model is proposed to study wetting transitions. The results of theoretical analysis are compared with those of experiment indicating that the proposed model can effectively predict the wetting transition. Furthermore, a numerical simulation based on the meso scale Lattice Boltzmann Method (LBM) is performed to study dynamic contact angles, contact lines, and local velocity fields for the case that a droplet displays on the micro structured surface. A spherical water droplet with r s = 15 m falls down to a biomimetic square-post patterned surface under the force of gravity with an initial velocity of 0.01 mโขs 1 and an initial vertical distance of 20 m from droplet centre to the top of pots. In spite of a higher initial velocity, the droplet can still stay in a Cassie state; moreover, it reaches an equilibrium state at t 17.5 ms, when contact angle is 153.16 which is slightly lower than the prediction of Cassie-Baxter's equation which gives CB = 154.40 .