Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete,mortar,stucco,and non-speciality grout.Dicalcium silicate (Ca2SiO4) is the primary constituent of a number of different types of cement.The β-Ca2SiO4 phase is metastable at room temperature and will transform into γ-Ca2SiO4 at 663K.In this work,Portland cement is annealed at a temperature of 950 K under pressures in the range of 0-5.5 GPa.The high pressure experiments are carried out in an apparatus with six anvil tops.The effect of high pressure on the obtaining nano-size β-Ca2SiO4(C2S) process is investigated by x-ray diffraction and transmission electron microscopy.Experimental results show that the grain size of the C2S decreases with the increase of pressure.The volume fraction of the C2S phase increases with the pressure as the pressure is below 3 GPa,and then decreases (P ＞ 3 GPa).The nano-effect is very important to the stabilization of β-Ca2SiO4.The mechanism for the effects of the high pressure on the annealing process of the Portland cement is also discussed.