In the paper, using the one-seed method and multiseed method separately, the hexahedral type-Ib diamonds are synthesized in a cubic anvil under high pressure and high temperature. This cubic anvil is of 550 mm hydraulic cylinder with the sample chambers of 14 mm or 26 mm in diameter under 5.6 GPa and 1200-1400 ℃. The FeNiMnCo alloy is chosen as catalyst. The high-quality abrasive diamonds each with a diameter of 0.9 mm are used as seed crystals. High purity-graphite powder (99.99%, purity) is selected as the carbon source. The effects of cavity size on the growth of hexahedral type-Ib Gem-diamond single crystal are studied carefully. The Relationship between oil pressure and synthesis pressure is obtained in our studies. When the pressure is transmitted the same distance, in the catalyst melt, the pressure loss is less than in the pressure transmitting medium. By expanding synthesis cavity size, the pressure transmission efficiency of the oil pressure increases significantly, which can be attributed to the transmission distance shortening in the pressure transmitting medium and transmission distance lengthening in the catalyst melt. Using the 14 mm synthesis cavity, by the one-seed method, the 5 mm grade diamond single crystals of cubo-octahedral shape are synthesized, but the 5 mm grade diamond single crystals of perfectly hexahedral shape could not be synthesized. Choosing the 14 mm synthesis cavity, by the five-seed method, the 3 mm grade diamond single crystals in the center each present a perfectly hexahedral shape, but each outside of the crystals exhibits a cubo-octahedral shape. According to the application requirement for the type-Ib hexahedral diamond single crystal with a size of 3.0-3.5 mm on an industrial diamond single crystal tool, the diamond single crystals of perfect hexahedral shape are synthesized by the multiseed method. Using the 26 mm synthesis cavity, many 3 mm grade diamond single crystals of perfectly hexahedral shape are synthesized in one synthesis cavity. In our studies, up to 14 diamond single crystals of perfect hexahedral shape are synthesized in one synthesis cavity by the multiseed method. We find that the uniformity of temperature field of the 26 mm synthesis cavity is better than that of the 14 mm synthesis cavity, so the 26 mm synthesis cavity is suitable for growing 3 mm grade diamond single crystals of perfect hexahedral shape by the multiseed method. In 35 h growth time, the overall growth rate of the 26 mm synthesis cavity (25.2 mg/h) synthesizing 14 diamonds in one time (9.4 mg/h) is 2.68 times that of the 14 mm synthesis cavity by five-seed method. Moreover, the Raman spectra of the synthesized high-quality hexahedral type-Ib diamond single crystals and natural diamond single crystal indicate that the structure and quality of the synthesized high-quality diamond single crystal is better than that of a natural diamond.