Large high-quality type IIa diamond crystals measuring up to 12 mm in diameter were successfully synthesized by the temperature gradient method at high pressure and high temperature, using high-crystalline-quality (001)-oriented seed crystals, and by controlling the temperature conditions with high precision. The X-ray projection topograph using a synchrotron radiation beam revealed that the large synthetic diamond crystals are characterized by high-crystalline-quality, having no dislocation or stacking fault in the (001) growth sectors in the upper part of the crystals grown on seeds. Diamond crystals containing a large defect free area of $5 \times 5$ mm 2 or more were successfully produced by controlling the temperature strictly on the low-temperature side in the synthesis region for growing type IIa diamond crystals to allow the (001) growth sectors to become dominant.