A large area (20 mm × 20 mm) of single-crystalline monolayer hBN without any microscopic defects (grain boundaries, twins, etc.) was successfully synthesized by MOCVD at little above the atmospheric pressure (770 torrs). It was primarily possible due to utilizing the large area and single-crystalline Cu as a substrate prepared by a contactless heat treatment method to prevent any heat-related stress to the specimen. With carefully controlled and well-organized experiments, it was found that a slow increase in temperature is more critical than long-time heat treatment. Secondly, heat treatment at 1083℃ after a quick temperature reduction from 1084℃ to 1083℃ within 1 min is another key condition to obtain a smooth surface for the growth of single-crystalline Cu without any defects. Microstructural studies of synthesized hBN were carried out by X-ray diffraction, optical microscopy, scanning electron microscopy, and transmission electron microscopy.