Abstract: Provided is an atomic layer deposition (ALD) apparatus that has an improved reactor and sample holder. The apparatus includes a reactor including an upper plate and a lower plate and accommodating a reaction chamber; and a sample holder supporting a sample loaded into the reaction chamber. The upper plate includes a bottom having a predetermined depth and a sidewall surrounding the bottom, and the bottom and the sidewall of the upper plate define the reaction chamber. At least one gas inlet and at least one gas outlet are installed at the sidewall of the upper plate. The sample holder includes a body and a cylindrical support member. The body has a support plate and a cylindrical support skirt. The sample is mounted on one side of the support plate and the support skirt extends from the other side of the support plate. Also, the support member is inserted in the body and supports the sample. The support plate includes a window that exposes the surface of the sample on which a thin layer is grown.