Disclosed is a fluorenylidene-diphenol-containing polyphenylene oxide which is defined by the following structural formula wherein D, X, Y, Z and b are defined in the specification. The fluorenylidene-diphenol-modified polyphenylene oxide resin or its prepolymer may be used for producing resin products with better thermal resistance, dielectric property, flame retardancy and lower thermal expansion. Accordingly, the resin products, such as prepregs, laminates or printed circuit boards, are suitable for use in electronic products with high speed and high frequency signal transmission to further improve the reliability, thermal resistance, and dimensional stability of the electronic products.