The conformational stability of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4], n = 3-8) under high pressure was investigated using Raman spectroscopy to reveal the preferential role of the alkyl-chain length (n) in high-pressure glass transition. To evaluate this, we determined the intensity ratio (r) and differences in the partial molar volume (ΔVtrans→gauche) between the whole trans and gauche conformers of the [Cnmim] cation using Raman intensities. Interestingly, both values were classified into a two alkyl-chain length region at the border of n = 5. The coulombic interaction (cation-anion interaction) for the conformational stability is the predominant factor below n = 5 (the cation-head portion: alkyl carbon number C < 5), and the alkyl-chain packing effect (cation-cation interaction) is the predominant factor above n = 5 (the cation-tail portion: C > 5). In combination with the conformational preference of the [Cnmim] cation under a high-pressure glassy state, the alkyl chain displays a preferential role, i.e., an increase in the gauche conformer of [Cnmim][BF4] adjusts to avoid crystallization (the conformational adjustment effect). In the presence of the coulombic interaction, the preferential role of the flexible alkyl chain is an important key to elucidate the mechanism of the complicated high-pressure phase transition behavior of ionic liquids.