Theoretical models, especially within the framework of Quantum Gravity, allow for the possibility of a velocity dispersion effect for photons of different energies traveling cosmological distances. Due to their fine‐scale time structure and their broad band emission, that is accurately modeled in this work, Gamma Ray Bursts could be good probes of such effect. GLAST will detect several GRBs per year at high energy where the effect could be detectable. © 2004 American Institute of Physics