In this study, the relationship between spatial position of diamond seed and growth mode has been investigated with an enclosed-type holder for single-crystal diamond growth using MPCVD epitaxial method. It demonstrates that there are mainly three different regions by varying the spatial position of seed. Owing to the plasma concentration occurs at seed edge, a relatively deeper depth is beneficial to transfer the plasma to the holder surface and suppress the polycrystalline diamond rim around the seed edge. However, the plasma density at the edge will be decreased drastically when the depth is too large, resulting in the growth of vicinal grain plane and the reduction of surface area. By adopting an appropriate spatial location, the size of a single-crystal diamond can be enlarged from 7×7×0.35 mm3 to 8.6×8.6×2.8 mm3 without the polycrystalline diamond rim.