The supersonic flow of nitrogen into a nanochannel is investigated using a three dimensional unstructured Direct Simulation Monte Carlo (U3DSMC) method. The U3DSMC code is validated by comparisons with previous 2D DSMC simulations of flows in micron-scale channels. Rectangular nanochannels with heights between 100 nm to 1000 nm, and aspect ratios L/H of 1, 10, 100 are used in the U3DSMC investigation. The Mach 5.9 freestream has a pressure of 0. 1atm and Knudsen numbers of 0.481, 0.962 and 4.81. The nanochannel walls are assumed to be diffusively reflecting at the freestream temperature. The simulations show the development of a disturbance region upstream from the inlet that depends on the Knudsen number. For the L/H = 10 and L/H = 100 nanochannels considered the velocity decreases from its freestream value velocity decreases from its freestream value and becomes subsonic inside the nanochannel. The temperature shows an enhancement region near the inlet while the density shows an enhancement region inside the nanochannel.

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