We use the recent measurement of the isotropic γ–ray background (IGRB) by Fermi LAT and analysis of the contribution of unresolved point γ–ray sources to IGRB to build constraints on the models of ultra-high cosmic rays (UHECR) origin. We also calculate the minimal expected diffuse γ–ray flux produced by UHECR interactions with an interstellar photon background. Finally, for the subclass of dark matter (DM) models with decaying weakly interacting massive particles (WIMP), we build constraints on the particle decay time using minimal expected contributions to the IGRB from unresolved point γ–ray sources and UHECR.