  In this work we study the constraints on dark matter (DM) annihilation/decay
from the Fermi-LAT Isotropic Gamma-Ray Background (IGRB) observation. We
consider the contributions from both extragalactic and galactic DM components.
For DM annihilation, the evolutions of extragalactic DM halos are taken into
account. We find that the IGRB constraints under some DM subhalo models can be
comparable to those derived from the observations of dwarf spheroidal galaxies.
We also use the IGRB results to constrain the parameter regions accounting for
the latest AMS-02 electron-positron anomaly. We find that the majority of DM
annihilation/decay channels are strongly disfavored by the latest Fermi-LAT
IGRB observation; only DM annihilation/decay to $\mu^+\mu^-$ may be valid.
