Fatigue crack growth (FCG) behavior of magnesium alloys at room temperature (20degreesC-55%RH) was investigated. Center cracked plate tension (CCT) specimens were used in the FCG tests for applying both tension-compression and tension-tension loading. The effects of R ratio and crack closure on fatigue crack growth behavior of extruded AZ61 and AZ91D magnesium alloys were investigated. From the results, it is found that stress ratio influences the threshold value Delta K-th for the materials used. The threshold value decreased with increasing stress ratio. The relationship between fatigue crack growth rate da/dN and stress intensity factor range Delta K for AZ61 was almost identical to that for AZ91D. The effective stress intensity factor range Delta K-eff values for R=-1, 0.1 and 0.5 were significantly low compared to Delta K values in the near-threshold region while the difference became smaller as the stress intensity factor range increased. This can be explained by the effect of crack closure, which is more pronounce in the near-threshold region. The effect of R ratio was diminished and all the growth curves laid on the same curve when the fatigue crack growth rate plotted as a function of Delta K-eff.