A study of strain relaxation in GaNxP1-x epilayers grown by chemical beam epitaxy, using a rf-plasma nitrogen radical beam source, tertiarybutylphosphine, and triethylgallium, is reported. Microcracks are observed in GaNxP1-x epilayers grown on GaP when the nitrogen composition is greater than 1.6%. Transmission electron microscopy results show that the tensile-strain relaxation in GaNxP1-x epilayers is initially relieved by microcrack formation without misfit dislocations. These microcracks penetrate through the interface, degrading the crystallinity of the GaP substrate. Microcracks formation can not be alleviated by adjusting the growth rate, growth temperature, V/III ratios, and forward plasma power, but they can be eliminated by reducing the growth area of the GaP substrate.