High current pulsed electron beam (HCPEB) treatment was conducted on an AZ91 cast magnesium alloy with accelerating voltage 27 kV, energy density 3 J/cm(2) and pulse duration 2.5 mu s. The surface microstructure was characterized by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope(SEM) equipped with energy dispersive spectrometer (EDS), and transmission electron microscope (TEM). The surface corrosion property was tested with electrochemical method in 3.5 wt.% NaCl solution. It is found that after 1 pulse of HCPEB treatment, the initial eutectic alpha phase and Mg17Al12 particles started to dissolve in the surface modified layer of depth similar to 15 mu m. When using 15 HCPEB pulses, the Al content in surface layer increased noticeably, and the phase structure was modified as composite nanostructures consisted of nano-grained Mg3.1Al0.9 domains surrounded by network of Mg17Al12 phase. The HCPEB treated samples showed an improved corrosion resistance with cathodic current density decreased by two orders of magnitude as compared to the initial AZ91 alloy. (C) 2014 Elsevier B. V. All rights reserved.