  Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in
the heavy metal / ferromagnet (HM/FM) bilayer systems. We observe the USMR in
the Pt/{\alpha}-Fe2O3 bilayers where the {\alpha}-Fe2O3 is an antiferromagnetic
(AFM) insulator. Systematic field and temperature dependent measurements
confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven
by the imbalance of creation and annihilation of AFM magnons by spin orbit
torque due to thermal random field. However, unlike its ferromagnetic
counterpart, theoretical modeling reveals that the USMR in Pt/{\alpha}-Fe2O3 is
determined by the antiferromagtic magnon number, and with a non-monotonic field
dependence. Our findings extend the generality of the USMR which pave the ways
for the highly sensitive detection of AF spin state.
