In this paper, we present our study on a new type of passive micromixer based on a washboard microfluidic configuration. Periodic geometrical barriers like washboard are built inside a microfluidic channel that alters the flow patterns transversely and vertically. The advantages of this type of mixer is its mixing barriers are at the bottom of the microfluidic channel, and it does not need a complex 2-D or 3-D configurations to perform mixing process. This micromixer can easily be fabricated by one step SU-8 photolithographic process and one molding process. Solutions to be squeezed vertically and laterally while encounter the periodic barriers. Thus, the laminar flow pattern is distorted to create mixing process. To study the mixing mechanism of the skew corrugated micromixer, we study the mixing efficiency of washboard structures at three different angles, including 30, 45, and 60 degrees. Finite element simulations are conducted to study the mixing pattern and efficiency. Simulation results suggested that a skew corrugated microchannel with 45^o angle can provide highest mixing efficiency, and a 95% mixing efficiency can be achieved within 8 stage within a 2.38 mm long microchannel.