Grafting method was used to modify polypropylene (PP) fiber to enhance the fiber reinforced concrete in this study. Polyacrylamide (PAM) was grafted onto PP surface by UV radiation using benzophenone (BP) as a photoinitiator. The microstructure of PP, and the interaction force between PP and the polar probe were observed with an atomic force microscopy (AFM). Experimental results showed that the roughness of PAM-grafted PP surface decreased significantly in comparison with that of the untreated PP surface. In addition, instead of smooth force curve as that of the untreated PP, the interaction between the probe and PAM-grafted PP surface appeared an oscillation. The above results gave the evidences that PAM had been successfully grafted onto the PP surface as discussed . The high hydrophilic property of PAM modifies the surface property of PP fiber and strengthens PP fiber reinforced concrete.