The Penetration Factor for slowly varying potential barriers in quantum mechanics is approximated by considering the tunneling probability through a barrier that changes gradually over a spatial scale. This method involves the concept of calculating the integral of the momentum of the particle within the barrier region, taking into account the variation in potential. The key idea is to use the WKB (Wentzel-Kramers-Brillouin) approximation, which is well-suited for barriers that vary slowly compared to the wavelength of the particle. In this approach, the transmission coefficient, which reflects the tunneling probability, is influenced significantly by the exponential decay of the wavefunction in regions where the potential energy exceeds the total energy of the particle. This approximation method provides a qualitative understanding of how particles can tunnel through slowly varying barriers, emphasizing the importance of barrier shape and width in determining tunneling probabilities.