Polycardanol was synthesized from cardanol, paraformaldehyde, p-toluenesulfonic acid, phosphoric acid (H 3 PO 4 ), and phosphorus pentoxide (P 2 O 5 ) via a two-step process. Results indicated that polycardanol is an acid with high molecular weight and can be self-crosslinked at high temperature. A modified WRP (MWRP) grafted by long chain can be obtained from the reaction between WRP and polycardanol. The sulfur content of MWRP is 0.27%, which is lower than that of WRP by 0.47%. The oxygen content of MWRP is higher by 13% than that of WRP. The phosphorus content of MWRP reaches 5.25%. The water contact angle of MWRP is 91.58, whereas that of WRP is 123.78. The properties of the WRP/NR and MWRP/NR composites were also investigated. MWRP/NR possesses higher tensile strength than WRP/NR because of the enhanced interfacial interaction between MWRP and the NR matrix. Post-treatment is also conducive for MWRP/NR to improve its tensile strength at high MWRP content.