Fatigue crack propagation tests of magnesium alloy were conducted under conditions of biaxial and uniaxial loading by using a cruciform specimen in a biaxial fatigue machine, in order to investigate the effect of non-singular stress cycling on the fatigue crack growth properties ΔKI -da/dN. The Magnesium alloys (AZ31B-O) used for this research are 2.5mm thickness plates. There are four different kinds of plates due to their heat treatment conditions. These conditions are (a) with no heat treatments (AZ31B-O), (b) 200-degree 2 hours (AZ31B-200), (c) 350-degree 2 hours (AZ31B-350), and (d) 430-degree 2 hours (AZ31B-430). From these comprehensive experiments, the remarkable effect was found in the specific biaxial load stress ratio σx0/σy0 on ΔKI -da/dN relation. When biaxial load stress ratio was 0.5, it turned out that the fatigue crack propagation rate of a magnesium alloy becomes very slow. Of course, in other biaxial load stress ratios, fatigue crack propagation velocity was influenced to some extent. It turned out that fatigue crack propagation rate becomes fast when a biaxial load stress ratio is minus, and it becomes slow when a biaxial load stress ratio is plus. Some discussion is made on the effect of microstructure on fatigue crack propagation of magnesium alloy in a biaxial stress field.