Reinforcing fibrous material

A reinforcing fibrous material having an improved adhesion, which consists essentially of a surface-treated molecularly oriented, silane-crosslinked ultra-high-molecular-weight polyethylene fiber, wherein, when the measurement is conducted under restraint conditions by using a differential scanning calorimeter, the crosslinked polyethylene fiber has at least two crystal melting peaks (Tp) at temperatures higher by at least 10.degree. C. than the inherent crystal melting temperature (Tm) of the ultra-high-molecular-weight polyethylene determined as the main peak at the time of the second temperature elevation, the heat of fusion based on these crystal melting peaks (Tp) is at least 50% of the whole heat of fusion, and the sum of heat of fusion of high-temperature side peaks (Tp1) at temperatures in the range of from Tm+35).degree.C. to (Tm+120.degree. C. is at least 5% of the whole heat of fusion, and wherein the surface-treated crosslinked polyethylene fiber has a smooth surface containing at least 8 oxygen atoms per 100 carbon atoms, as determined by the electron spectroscopy for chemical analysis (ESCA), with the width of surface cracks in the orientation direction controlled below 0.1 .mu.m.