The　direct　growth　of　wafer-scale　single　crystal　two-dimensional　(2D)　hexagonal　boron　nitride　(h-BN)　layer　with　a　controllable　thickness　is　highly　desirable　for　2D-material-based　device　applications.　Here,　for　the　first　time,　a　facile　submicron-spacing　vapor　deposition　(SSVD)　method　is　reported　to　achieve　2-inch　single　crystal　h-BN　layers　with　controllable　thickness　from　monolayer　to　tens　of　nanometers　on　the　dielectric　sapphire　substrates　using　a　boron　film　as　the　solid　source.　In　the　SSVD　growth,　the　boron　film　is　fully　covered　by　the　same-sized　sapphire　substrate　with　a　submicron　spacing,　leading　to　an　efficient　vapor　diffusion　transport.　The　epitaxial　h-BN　layer　exhibits　extremely　high　crystalline　quality,　as　demonstrated　by　both　a　sharp　Raman　E-2g　vibration　mode　(12　cm(-1))　and　a　narrow　X-ray　rocking　curve　(0.10　degrees).　Furthermore,　a　deep　ultraviolet　photodetector　and　a　ZrS2/h-BN　heterostructure　fabricated　from　the　h-BN　layer　demonstrate　its　fascinating　properties　and　potential　applications.　This　facile　method　to　synthesize　wafer-scale　single　crystal　h-BN　layers　with　controllable　thickness　paves　the　way　to　future　2D　semiconductor-based　electronics　and　optoelectronics.