Asymmetric light propagation is crucial to the development of optical-based functional components in nanophotonics. Diverse configurations and structures have been proposed to allow asymmetrical propagation of photonic signal, but on-chip integration is difficult to achieve due to their complex structure and/or relatively large footprint. Here we report the first design and realization of asymmetric light propagation in single semiconductor nanowires with a composition gradient along the length. We show the asymmetric nanowire waveguides can be synthesized using a simple thermal evaporation and vapor transport approach without involving complicated and costly fabrication processes. Our studies demonstrate the asymmetric nanowire waveguides offer some significant advantages over previous designs, including ultra-low operation power, tunable working wavelength and nanoscale footprint, making them attractive building blocks for integrated photonic circuits.