A method of producing a diamond-on-silicon wafer comprising: attaching diamond particles to a surface of a silicon wafer having an oxygen concentration of 5 × 10 17 atom/cm 3 or less, and then growing a diamond layer on the silicon wafer by chemical vapor deposition using the diamond particles as nuclei, wherein a maximum particle size of the diamond particles defined as the maximum length of the major axis of the diamond particle in the diamond layer is 2.0 μm or less, wherein the maximum particle size of the diamond particles in the diamond layer was determined by observing three regions of the wafer that are 10 μm × 10 μm in size under an optical microscope, wherein the three regions are centered at three points, that is, the center of the wafer and the two intersection points between a wafer diameter and the circumference of a circle having a radius which is 95 % of the radius of the wafer radius, the circle being centered at the wafer center, and wherein the maximum particle size of all the diamond particles in the three regions is defined as the maximum particle size, wherein a thickness of the diamond layer is 10 μm or more, wherein the oxygen concentration means the mean value of the oxygen concentrations in a thickness direction of the silicon wafer, measured by FT-IR spectroscopy according to Old ASTM F121-1979, wherein either after a diamond particle-containing solution is applied to the surface of the silicon wafer or after the silicon wafer is dipped in a diamond particle-containing solution, heat treatment is performed on the silicon wafer, thereby attaching the diamond particles to the surface of the silicon wafer, wherein an average particle size, measured by a laser diffraction particle size analyzer and calculated based on JIS 8819-2, of the diamond particles in the diamond particle-containing solution is 50 nm or less. attaching diamond particles to a surface of a silicon wafer having an oxygen concentration of 5 × 10 17 atom/cm 3 or less, and then growing a diamond layer on the silicon wafer by chemical vapor deposition using the diamond particles as nuclei, wherein a maximum particle size of the diamond particles defined as the maximum length of the major axis of the diamond particle in the diamond layer is 2.0 μm or less, wherein the maximum particle size of the diamond particles in the diamond layer was determined by observing three regions of the wafer that are 10 μm × 10 μm in size under an optical microscope, wherein the three regions are centered at three points, that is, the center of the wafer and the two intersection points between a wafer diameter and the circumference of a circle having a radius which is 95 % of the radius of the wafer radius, the circle being centered at the wafer center, and wherein the maximum particle size of all the diamond particles in the three regions is defined as the maximum particle size, wherein a thickness of the diamond layer is 10 μm or more, wherein the oxygen concentration means the mean value of the oxygen concentrations in a thickness direction of the silicon wafer, measured by FT-IR spectroscopy according to Old ASTM F121-1979, wherein either after a diamond particle-containing solution is applied to the surface of the silicon wafer or after the silicon wafer is dipped in a diamond particle-containing solution, heat treatment is performed on the silicon wafer, thereby attaching the diamond particles to the surface of the silicon wafer, wherein an average particle size, measured by a laser diffraction particle size analyzer and calculated based on JIS 8819-2, of the diamond particles in the diamond particle-containing solution is 50 nm or less.