With the development of microfluidic systems, there is a growing interest in micro scale laminar flow mixing. In this work, the fluid rotating angle and mixing efficiency in a micromixer with patterned grooves are studied as a function of the dimensions of the microstructure by numerical simulation. We found that mixing efficiency does not always increase with higher fluid stream rotation in the microchannel. High groove aspect ratios are not advantageous to fluid rotation. Experiments on mixture of two fluids were done on a micromixer fabricated in PDMS by replica molding. An 85% mixing efficiency was obtained in a 30mm long mixing channel with two dyed liquids.