In order to reduce driving power consumption, we propose and fabricate a new structure of asymmetric SOI 1${ imes}$2 thermo-optic switch that has a back side silicon trenched structure. Compared to conventional SOI thermo optic switches without heat sink structure, it shows an improvement of switching power reduction from about 4 watt to 1.8 watt without sacrificing cross talk of about 20 ㏈ at the light wavelength of 1.55 $mu extrm{m}$. Here we also described the main design consideration and fabrication procedure for the proposed device.