An extensive research programme was set up to investigate the efficiency of scrap rubber particles on strength of cementitious materials. The index of strength loss rate of paste and mortar sample caused by increasing 1% volume rubber particles was proposed to analyze the efficiency of rubber particles in cementitious materials. And the corresponding mechanism was also discussed in this paper. Results indicate that in the investigated area, the loss rate in compressive strength both for paste and mortar almost ranges from 2% to 5% caused by increasing 1% rubber particles depending on the total volume of rubber particle added into sample. However, the loss rate in flexural strength of mortar by increasing 1% volume rubber particles is obviously different from that. Three roles of scrap rubber particles played in cementitious materials, named as deformation effect, equivalent-pore effect and hydrophobic effect, are responsible for the efficiency of scrap rubber particles on strength of cementitious material. The efficiency of rubber particles on strength of paste and mortar differs from each other due to the different microstructure between paste and mortar.