A $1{ imes}16$ thermo-optic switch with small excess loss using multimode interference(MMI) couplers is designed, fabricated, and measured. This paper introduces the proposed $1{ imes}16$ thermo-optic switch, and discusses the measurement results. The $1{ imes}16$ thermo-optic switch is farmed as 4-stage which consists of 15 unit devices. The unit devices are the $2{ imes}2$ thermo-optic switches with Mach-Zehnder interferometer(MZI) structure. The characteristics of the $1{ imes}16$ thermo-optic switch depends strongly on each unit device. The unit deviceconsists of two 3-dB general interference MMI couplers and two single mode waveguide arms as a phase shifter. First of all, the 3-dB optical splitter and $2{ imes}2$ MZI thermo-optic switch have been tested to confirm the characteristics of the unit devices of the $1{ imes}16$ MZI thermo-optic switch. Using the measurement results of the unit devices, the $1{ imes}16$ MZI thermo-optic switch can be produced with better characteristics. The resultant structure of the MMI coupler with the optical light source of wavelength of 1550nm for the $1{ imes}16$ thermo-optic switch is that the width and the optimized length are $25{mu}m;and;1580{mu}m$, respectively. The smallest excess loss fur the unit device is -0.5dB and the average excess loss is -0.7dB.