The authors study the electrical transport properties of atomically thin individual crystalline grains of MoS2ﾠwith four-probeﾠscanning tunneling microscopy.ﾠTheﾠmonolayerﾠMoS2ﾠdomains are synthesized byﾠchemical vapor depositionﾠon SiO2/Si substrate. Temperature dependent measurements on conductance andﾠmobilityﾠshow that transport is dominated by an electron charge trapping and thermal release process with very lowﾠcarrier densityﾠandﾠmobility.ﾠThe effects of electronicﾠirradiationﾠare examined by exposing the film toﾠelectron beamﾠin theﾠscanning electron microscopeﾠin an ultrahigh vacuum environment. Theﾠirradiationﾠprocess is found to significantly affect theﾠmobilityﾠand theﾠcarrier densityﾠof the material, with the conductance showing a peculiar time-dependent relaxation behavior. It is suggested that the presence of defects in active MoS2ﾠlayer and dielectric layer create charge trapping sites, and a multiple trapping and thermal release process dictates the transport andﾠmobilityﾠcharacteristics. Theﾠelectron beamﾠirradiationﾠpromotes the formation of defects and impact the electrical properties of MoS2. Our study reveals the important roles of defects and theﾠelectron beamﾠirradiationﾠeffects in the electronic properties of atomic layers of MoS2.