•The initial growth of heteroepitaxial diamond on Ir(001)/MgO(001) was investigated.•Diamond grains gradually grew larger in diameter and thickness in a three-dimensional manner.•In-plane orientation of diamond grains had improved significantly before coalescence.•The proportion of diamond component gradually increased as diamond grain grew.•Initial growth of heteroepitaxial diamonds from nucleation-treated surfaces to film formation through coalescence was modelled. It is important to clarify the mechanism of heteroepitaxial diamond growth, focusing on the change from three-dimensional to two-dimensional growth through coalescence, including dislocation reduction, impurity doping, etc. A detailed investigation was made of the relationship between the surface morphology and the crystalline quality of diamonds during the initial and early stages of their growth. An Ir (001) layer formed on a MgO (001) substrate was treated by ion irradiation through direct-current discharge. Diamond was then grown on the ion-irradiated surface for a short time by using a microwave plasma CVD (MPCVD) process. Surface and cross-sectional scanning electron microscopy (SEM) and atomic-force microscopy (AFM) were used to investigate how the diamond grains grew to form a film. The resulting diamond grains and film were also characterized by means of X-ray diffraction (XRD) and Raman spectroscopy. The diamond grains randomly united to form a film, their in-plane orientation improved until coalescence was complete, and the proportion of diamond component gradually increased. Finally, we developed a model for growth of heteroepitaxial diamond grains based on our detailed observations.