Provided is a MgO single crystal for obtaining a magnesium oxide (MgO) single crystal deposition material which is prevented from splashing during the vapor deposition in, e.g., an electron beam deposition method without reducing the deposition rate, and for obtaining a MgO single crystal substrate which can form thereon, e.g., a superconductor thin film having excellent superconducting properties. A MgO single crystal having a calcium content of 150.times.10.sup.-6 to 1,000.times.10.sup.-6 kg/kg and a silicon content of 10.times.10.sup.-6 kg/kg or less, wherein the MgO single crystal has a variation of 30% or less in terms of a CV value in detected amounts of calcium fragment ions, as analyzed by TOF-SIMS with respect to the polished surface of the MgO single crystal. A MgO single crystal deposition material and a MgO single crystal substrate for forming a thin film obtained from the MgO single crystal.