Mg-Dy alloys show promising for being developed as biodegradable medical applications. It is found that the hydride DyH₂ could be formed on their surface during their sample preparations. The formation of hydrides in Mg-Dy alloys is affected by the content of Dy and heat treatments. The amount of hydride DyH₂ increases with the increment of Dy content. Their formation mechanism is attributed to the surface reaction of Mg-Dy alloys with water. The part of Dy in solid solution plays an important role in influencing the formation of hydrides.