In order to investigate the effect of humidity on initiation and propagation properties of a fatigue crack of 18%Ni maraging steels with different grain size and hardness, rotating bending fatigue tests were carried out in relative humidity of 25% and 85%. Fatigue strength was largely decreased by high humidity and the decrease in fatigue strength due to high humidity was increased with increases in grain size and specimen hardness. In high humidity, both of initiation and propagation of a crack were accelerated. The acceleration of a crack growth was caused by intergranular crack. Although the proportional relation between fatigue limit and hardness held up to HV600 in high humidity of 85%, the limiting hardness in the relation increased to HV 750 in low humidity of 25%. The decrease in fatigue strength due to high humidity was suppressed by double aging, which was aged at 673K for 3.6Ks after aging at 753K for 150ks.