  We uncover an interesting phenomenon that neutrino flavor transformation in
slowly varying matter density imitates almost exactly that of vacuum neutrino
oscillation under suitably chosen experimental parameters. It allows us to have
relatively large CP violating measure \Delta P \equiv P(\nu_{\mu} \to \nu_e) -
P(\bar{\nu}_{\mu} \to \bar{\nu}_e) which is essentially free from matter effect
contamination. We utilize this phenomenon to design a low-energy long-baseline
neutrino oscillation experiment to measure the leptonic CP violating phase.
