The effects of two kinds of heat treatments T4 (solution treatment) and T6 (aging treatment) on the corrosion behaviors of Mg-3Zn magnesium alloy were studied by electrochemical measurements and scanning electron microscopy (SEM). It is found that zinc element enriches along grain boundaries to exhibit a network microstructure for both T4- and T6-treated alloy. For T6 treatment, larger MgZn particles form mainly on grain boundary and fine MgZn particles precipitate on matrix. Compared with cast alloy, T4 treatment could decrease the amounts of MgZn particles, and decrease the zinc content of zinc-rich net-segregation. Electrochemical measurements show that T4 treatment increases the corrosion resistance while T6 treatment decreases the corrosion resistance of Mn-3Zn alloy.