Large-scale diamond nanocrystals of different sizes were synthesized by 60 keV 40Ar+ irradiation on amorphous carbon. Investigated by high-resolution transmission electron microscopy, energy diffraction X-ray spectrum, electron diffraction and Raman spectrum, the diamond crystallites embeded in graphitic film have a high nucleation density (about 1013/cm2) and can grow to large sizes even of the micrometer order. The mechanism of phase transition is discussed preliminarily.