The dynamic deformations at high temperatures of Al-3 wt%Mg alloy and Al-3 wt%Mg/B4C composites with different volume fractions and particle sizes were studied using a dilatometer deformation instrument and a split Hopkinson pressure bar operating at strain rates of 10–1000 1/s. A comprehensive analytical procedure was developed to correct the effects of adiabatic heating, friction at interface of the specimen and bars, and strain rate variation, on flow stress curves. Then based on corrected data, a physical based constitutive equation was developed for modeling and prediction of flow stress. It was observed that composites in comparison with single phase alloy, after initial straining, showed lower hardening rate which is unexpected. EBSD micrographs and finite element analysis were used to investigate microstructural evolution and deformation condition around particles. It was concluded that particle fracture during deformation which is more expectable in larger particles, and also higher adiabatic heating in composite and not recrystallization related phenomena, are the main reasons for softening of stress flow curves at large deformation.