A diamond single crystal substrate manufacturing method for growing by vapor-phase synthesis a single crystal from a diamond single crystal seed substrate, comprising etching away by reactive ion etching, prior to single crystal growth, at least 0.5 .mu.m and less than 400 .mu.m, in etching thickness off the surface of the seed substrate which has been mechanically polished, thereby removing from the surface of the seed substrate the work-affected layers caused by mechanical polishing; and growing then a single crystal thereon. The manufacturing method provides a diamond single crystal substrate having a high quality, large size, and no unintentional impurity inclusions, and suitable for use as semiconductor materials, electronic components, optical components or the like.