A novel Mach-Zehnder Interferometer (MZI)-based planar waveguide sensor is introduced. The microbending waveguides are used in the two arms of the MZI. The sensitive region and the influence of the microbending shape to the sensitivity of the sensor are studied with three-dimensional beam propagation method. The amplitude and period of a sin microbending structure are optimized with a gradient-based search algorithm. The sensitivity of the novel sensor is shown to be much better than that of a conventional MZI-based planar waveguide sensor.