This project studies the effects of imposing two external potentials, namely harmonic oscillator and Yukawa potentials, in nano-slit pores on the structure and layering phase transition of parallel hard cylinders. In addition, the layering structure of this kind of fluid at a single wall is obtained and explained using the concept of entropy and energy effects. For this purpose, the density functional theory (DFT) based on Onsager’s theory is used. As expected, the values for the contact density at the walls and the oscillations are found to depend greatly on the type of external potential applied. In fact, they are larger for the Yukawa potential than they are for the harmonic oscillator. Finally, the first order 8-9 layering phase transition is studied for the two different external potentials and the results are compared with those for hard walls. The shape of the coexistence curve and the thermodynamic states in which the transition occurs are found to vary in response to the type of field imposed.