The grafting polymerization of styrene on the surface of commercial available vulcanized acrylonitrile butadiene rubber (NBR) latex was initiated by the heat decomposing cleavage of dormant groups of semipinacol (SP) immobilized firstly under UV irradiation. Extremely high grafting efficiency in the range from about 90% to nearly 100% could be achieved. Certain factors affinitive to the grafting polymerization, such as the concentration of monomer, the UV irradiation time taking on the immobilization of SP groups, and concentration of NBR latex containing SP groups, were investigated. It was noted that there was the highest conversion when the weight ratio of NBR and St was 4:6; the grafting polymerization had a constant grafting efficiency that was almost not affected by polymerization time and the ratio of NBR-SP/St. Furthermore, the morphologies of the thermal compression molded composites of NBR/NBR-g-PSt were observed by TEM and the thermal properties as well as tensile behaviors were examined. Obviously rubber-plastic transition could be displayed with the increasing of grafting yield. When the grafting yield was controlled at about 50%, the strength of modified rubbers increased significantly without losing its elongation.