Three iron-silicon alloys (Fe85Si15, Fe71Si29, and ϵ-FeSi) have been studied in a diamond anvil cell at room temperature up to 55 GPa by in situ energy-dispersive X-ray diffraction techniques. A body centered cubic (bcc) to hexagonal close packed (hcp) phase transformation in Fe85Si15 began at 16 GPa and was completed by 36 GPa. No phase transformations were observed in either Fe71Si29 or ϵ-FeSi at high pressures, even when laser-heated to about 2000 K. The isothermal bulk modulus (K0T) of hcp-Fe85Si15 is 141 (¿10) GPa with K'0T = 5.70(¿0.60) and V02 = 6.882(¿0.031) cm3/mol (per molar atom). The K0T of Fe71Si29 is 199.0 (¿5.3) GPa with K'0T = 5.66(¿0.61) and V0 = 6.887(¿0.014) cm3/mol, and the K0T of ϵ-FeSi is 184.7 (¿3.9) GPa with K'0T of 4.75 (¿0.37) and V0 = 6.790(¿0.007) cm3/mol. Our study indicates that the substitution of Si into iron would lower the density of iron, but significantly changes its compressibility neither in the bcc phase, nor at high pressures in the hcp phase. Upon comparison with the Preliminary Reference Earth Model, the calculated equations of state (EOS) of hcp-Fe85Si15, using the Mie-Gr¿neisen EOS, indicate that an outer core containing about 8--10 wt.% Si and inner core containing about 4 wt.% Si in iron would satisfy the seismological constraints. Addition of silicon into iron increases the bulk sound velocity of iron, consistent with silicon being a light element in the Earth's core.