The C-V characteristic of a silicon p-n diode grown by molecular beam epitaxy (MBE) technique was carefully studied in the case that the space charge region reached the edge of MBE layer. Based on the theoretical analysis and experimental measurement, the impurity concentration in unintentionally doped n-type MBE layer was determined to be 8.0×1014cm-3. This method is shown to be a new way to find out impurity concentrations in supper-thin semiconductor layers.