Fatigue crack propagation tests of magnesium alloy were conducted under conditions of biaxial and uniaxial loading by using a cruciform specimen with the biaxial fatigue tester. The purpose of this study is to investigate the effect of biaxial stress on the fatigue crack growth properties Delta K(I)-da/dN. From these comprehensive experiments, the remarkable effect was found in the specific biaxiality ratio sigma(0)(x)/sigma(0)(y) on the Delta K(I)-da/dN relationship. When biaxiality ratio was 0.5, it turned out that the fatigue crack propagation velocity of a magnesium alloy becomes very slow. In other biaxiality ratios, fatigue crack propagation velocity was influenced in some extent. Fatigue crack propagation velocity decreases a little as the stress which acts in parallel with a crack increases.