The method of examining micro-pores in sulphoaluminate MDF cement by means of the Positron Annihilation Technique (PAT) was discussed.The results show that the intensity I2 of positrons annihilation with moderate life-span (τ2≈330Ps) increases regularly with the addition of the numbers of micro-pores (less than 250(A)). Therefore, the value of I2 may be used to opitomize the compactness and integrity of the MDF cement structure. At the same time, the experiment result of PAT was compared with that of Mercury Intrusion Porosimeter (MIP).