To develop a new practical method of purifying and recycling ionic liquids, we performed direct microscopic observations and in situ crystallization of low-melting ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), in detail by high pressure Raman spectroscopy. Compression of [BMIM][PF6] was measured under pressures up to about 2.0 GPa at temperatures 293-353 K by using a high pressure diamond anvil cell (DAC). At room temperature, with pressure increasing, the characteristic bands of [BMIM][PF6] displayed nonmonotonic pressure-induced frequency shifts, and [BMIM][PF6] experienced the liquid solid phase transition at about 0.50 GPa. In separate experiments, in situ crystallization of low-melting ionic liquid [BMIM][PF6] were also measured at various P T regions, in order to improve the understanding of its stability limits. Finally, the T versus P phase diagram of [BMIM][PF6] was constructed, and it showed that the melting point was an increase function of pressure. It was also indicated that the structure changes in the crystalline and liquid states under high pressure might also be associated with conformational changes in the butyl chain. Pressure-released Raman spectra also showed that the phase transition of [BMIM][PF6] was reversible.