The high-pressure (HP) phase transition and conformational change of n-octane (hereafter abbreviation as n-C-8) up to 15.3 GPa were studied using Raman spectroscopy to investigate the relationship between the HP phase state and the alkyl chain length of n-alkanes. The Raman spectral analysis of n-C-8 indicated that the liquid-solid transition (solidification) occurs at similar to 0.9 GPa and that the corresponding transition pressure of n-alkanes depends on their density. Further pressurization at similar to 4 GPa increased the population of the gauche conformer, while the solid (order)- amorphous transition occurred at similar to 6 GPa along with a change in the full width at half maximum of the ruby R-1 fluorescence line. The comparison of our findings with previously reported results suggested that the even-odd effect in the HP phase transition after solidification of n-alkanes appears between n-C-7 and n-C-8 as their HP phase transition up to similar to 15 GPa was different.