Switchable electrical and thermal conductivities are desirable in many applications such as automatic regulation of building temperature, circuit protection, etc. In this paper, we study the electrical and thermal conductivities of multi-walled carbon nanotube (MWCNT)/DI-water composites via first order phase transition. We demonstrate that, with SDBS functionalized MWCNTs, the composites show unique positive temperature coefficient (PTC) electrical switching and negative temperature coefficient (NTC) thermal switching properties. Around 0 degrees C, the corresponding contrast ratio of electrical and thermal conductivities reaches 1250 and 3.58 times respectively.