The fatigue crack growth behavior of magnesium single crystal has been investigated in laboratory air at room temperature. Four types of compact specimens with different notch orientations were prepared from magnesium single crystal made by the Bridgeman technique. In the case of A-specimen which have notch plane and direction are (11 (2) over bar0) and [1100] respectively, fatigue crack propagates parallel to notch. In the case of B-specimen with (1100)[11 (2) over bar0] notch, fatigue crack propagates inclined 30 degree from initial notch. In both case, fatigue crack direction is [1 (1) over bar 00] and their fracture surfaces are similar. From TEM observation, it is found that pyramial slips with (c+a) Burgers vector occures at the crack tip. When a D-specimen which notch is normal to basal plane, a fatigue crack propagates [0001] at low stress intensity factor range. At high stress intensity factor range, the crack propagates propagates parallel to basal plane. In the case of F-specimenwith notch parallel to basal plane, a fatigue crack propagates through straight route with some {10 (1) over bar2} twin. Fatigue crack growth rate, da/dN of A-and B- specimen is lower than F-specimen with crack parallel to basal plane.