The block copolymers of poly(ether ether ketone) (PEEK) and poly(aryl ether sulfone) containing biphenylene moiety (PEBS) were synthesized by a condensation reaction of fluorine-terminated PEEK oligomers and hydroxy-terminated PEBS oligomers. The content of PEBS in the copolymers was varied from 10 to 37 wt%, and the effect of the compositional variation on the properties was investigated. Microphase separation was not observed and the copolymer was a homogeneous system having good compatibility. The crystal structure of the copolymers is rhombic, equal to PEEK. Although the degree of crystallinity of the copolymers decreased with the increase of PEBS content, the glass transition temperature (T-g) rose greatly, and superior heat resistance and good mechanical properties at high temperature were obtained. Analyses of the isothermal crystallization kinetics of the copolymers provided an Avrami’s exponent (n) of 2.0.