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In this work, L-phenylalanine crystals are subjected to hydrostatic pressure up to 5 GPa by means of a diamond anvil cell, using the 1:1 FC70-FC77 FluorinertTM mixture as pressure transmitting medium (PTM). Raman spectroscopy is used as a probe, providing crucial information on the vibrational properties and the structural stability of the system under pressure. The Raman spectrum of the studied material is very rich, owing to the low symmetry of the phenylalanine molecule and the monoclinic crystal structure. The intermolecular (lattice) modes appear in the low frequency region (<250 cm-1), while the intermediate frequency region (300-1700 cm-1) contains the backbone, aromatic and sidechain vibrational modes. Finally, the high frequency region (2600-3300 cm-1) is dominated by C-H and N-H stretching vibrations and combination modes. Pressure application causes the shift of the vast majority of the observed Raman peaks to higher energies, their broadening (particularly, after the solidification of the PTM at ~1 GPa), as well as changes in their relative intensities. The pressure dependence of the frequencies of all the Raman peaks is quasi-linear up to ~2 GPa. However, for higher pressures, significant changes occur with respect to the number of the Raman peaks, their frequencies and pressure slopes in both the intermolecular and the intramolecular spectral region. The observed changes signal a structural phase transition of the crystalline L-phenylalanine accompanied by molecular conformation modifications. The new high pressure phase is stable to pressure application at least up to 5 GPa, while the observed phase transition is reversible and the initial phase is recovered during the downstroke experiments below 2 GPa.