Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause rupture. In order to understand the crack initiation and propagation phenomena, non-destructive evaluation methods that can be correlated to in-situ measurements around the crack tips are necessary. In the present work, we developed a scanning Hall probe microscope (SHPM) equipped in a GaAs film sensor to observe fatigue cracks at room temperature in air while they were growing. Medium carbon low alloy steels specimens (JIS S45C) were used in the experiments. Only the area around the crack tip was magnetized and the changes at the crack tips were observed.