In this paper we investigate the cold to hot phenomena at the nano step geometry. This geometry is applied in nano devices where straight channels with various diameters are connected to each other. The DSMC approach has been used to model nitrogen gas through this geometry. We show that there is a thermal separation zone in which heat lines deviate their direction from the normal direction. This zone can affect total gas behavior. The results show that big thermal separation zone can force heat lines to move from cold regions to hot ones. To understand the details of this process, the constitutive laws of the linearized Boltzmann theory has been used. The results of this theory show good agreement with DSMC results at small Knudsen numbers. This theory states that there are two main factors, i.e., Fourier effect and second derivative of velocity which determine the heat lines behavior. The balance between these two terms determines the heat direction. Our results show that second derivative of velocity is more dominant where the temperature gradient is weak. It means that if the differences between gas and wall temperature is low, the Fourier term will have little effect and second derivative of velocity determines the heat lines. This causes cold to hot heat transfer. The effect of Kn number and inlet temperature have been investigated as well. We will show that cold to hot heat transfer can occur only at a low Kn.

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