Mechanical behavior of processed AZ91D magnesium alloy by equal channel angular extrusion during semi-solid isothermal compression was investigated using semi-solid isothermal compression test. The results show that there are four stages, drastic increase of true stress, drastic falling of true stress, steady stage and increase of true stress. With the increase of holding time or deformation temperature, required true stress for obtaining same true strain falls evidently and peak stress and steady stress also fall in true stress-strain curve. With increasing strain rate, steady stress also increase. Peak stress will increase or decrease with the sudden increase or fall of strain rate. During semi-solid isothermal compression, plastic deformation of semi-solid billet mainly depends on plastic deformation of solid phases and their rotation and sliding. Coarse grains occur in upper position adjacent to compression die’s surface and fine grains occur in the central and free surface positions of billets.