In the paper, under 5.6 GPa and 1250−1450 ℃, the Ib-ype diamond single crystals chosen as the seed crystals with different sizes, are synthesized in a cubic anvil at high pressure and high temperature. High-purity Fe-Ni-Co solvents are chosen as the catalysts. High-purity graphite powder (99.99%, purity) is selected as the carbon source. Hexahedral abrasive grade high-quality diamonds of 0.8 mm, 1.5 mm or 2.2 mm in diameter are chosen as seed crystals. The effects of seed crystal size on the growth of gem-diamond single crystal are studied in detail. Firstly, the influence of the change of seed size on the cracking of diamond single crystal is investigated. The crystal growth law of increasing the probability of cracking crystal with larger seed crystal is obtained. It can be attributed to the following two points. i) The residual cross section at the separation of the main crystal from the larger seed crystal is too large, thus reducing the overall compressive strength of the crystal. ii) The growth rate of the diamond crystal synthesized by larger seed crystal is too fast, which leads to the increase of impurities and defects and the decrease of compressive strength of the crystal. The decrease of crystal compressive strength leads to cracks in diamond crystals during cooling and depressurizing. Secondly, in the growth time of 25 hours, the relationships between the growth time and the limit growth rate of the diamond single crystals synthesized by choosing three sizes of seed crystals are investigated. The results show that the high-quality single crystal synthesis efficiency can be improved and the synthesis period can be shortened by selecting large seed crystals. This is because the size of the seed crystal becomes larger at each stage of crystal growth, resulting in the enhancement of the ability of diamond single crystal to receive carbon, so that high-quality diamond single crystals can be grown at a faster growth rate. Thirdly, with the help of scanning electron microscope or optical microscope, we calibrate the surface morphologies of diamond single crystals grown with different-size seed crystals. Using the seed crystals of 0.8 mm, 1.5 mm or 2.2 mm in diameter, high-quality diamond single crystals with smooth surfaces can be synthesized. However, with the increase of seed crystal in size, the surface flatness of the grown crystals tends to decrease and the possibility with which surface defects occur and string inclusions increase. The growth rate of high-quality diamond single crystals grown with larger seed crystals must be strictly controlled. Finally, the N impurity content values of diamond single crystals grown with different seed crystals in size are characterized by Fourier transform infrared measurement. The results show that the N impurity content of the crystal increases with the diamond growing rapidly by selecting larger seed crystal.