Owing to its strong B–N covalent bonds and inferior dissolution–precipitation characteristics in selected liquid phases, densification of h-BN ceramics is generally achieved via pressure-assisted sintering techniques. Herein, high-density h-BN ceramics were fabricated by a secondary cold isostatic pressing (CIP(II))-assisted pressureless sintering process using commercial micron-sized h-BN powder as the raw material and deionized water as a green lubricant additive. The addition of water significantly promotes the particle sliding and rearrangement of lamellar structured h-BN during the forming process, and the relative density of the green body can reach 94.60%. A secondary cold isostatic pressing applied to the dried green body can effectively suppress cracking caused by water evaporation and further increase the relative density to 98.10%. The use of low-oxygen raw powder effectively inhibits void formation induced by volatilization, reduces density loss after high-temperature sintering, and enhances the thermal conductivity of pressureless-sintered h-BN ceramics. The internal flake-like h-BN grains retained a highly oriented layered structure, endowing the ceramic with excellent in-plane thermal conductivity (up to 57.16 W·m⁻¹·K⁻¹). This study offers an alternative approach for the industrial-scale production of high-density h-BN ceramics through pressureless sintering.