The thermal management becomes more significant because of the continuing shrinkage of electronics devices. The thermal MEMs devices are the major topics in the thesis. The thermo-optic and thermo-electro devices are involved. The Mach-Zhender type thermo-optic devices are used for the application of switch and modulator in optical communication. The thermo-electro devices on the flexible substrate can be used for measuring temperature of non-planar objects. The main issue of a Mach-Zherder interferometer based thermo -optic switch is that it needs large heating power to get adequate phase shift for switching. This is due to the nature of silicon substrate which possesses very high thermal conductivity. A device structure of etching in a silicon groove underneath the heated arm of waveguide was to suppress the heat loss through the bulk silicon substrate. This study shows that the optimum design of the device is obtained when the silicon etched groove is located asymmetrically with respect to the two waveguide brances. Most of temperature sensors are made in planar type because of the substrate. To realize non-planar temperature measurement the devices are needed to fabricate on flexible substrate. The thesis presents the fabrication and characterization of a three-dimensional (3D) thermopile; consists of 64 Cu-Ni series connected thermocouples on polyimide (PI) flexible substrate. Using wet etching to etch through 25 μm PI, the cold and hot junctions of thermocouples are formed on the top and bottom surface of PI substrate. This 3D layout design differentiates its innovative uniqueness from the traditional 2D planar thermopiles that have both hot and cold junctions on the same plane. The experimental studies on the PI etching with respect to the concentrations of KOH and C2H7NO in the etching solution conclude that the optimal composition of the etchant is 9M KOH with 1M C2H7NO and etched at 80 oC. A measured sensitivity of 0.44mV/K is realized in the fabricated device. The temperatures measured by the 3D thermopile are close to those obtained with a digital thermometer, demonstrating that 3D flexible thermopiles has great potential to provide low cost thermal sensor.