Highly ordered two-dimensional (2D) mesoporous silicoboron carbonitride (SiBCN) materials were prepared by a double nanocasting approach using mesoporous SBA-15 silica as starting template. The latter was converted into its negative replica CMK-3 carbon template, which was subsequently impregnated with a boron-modified polysilazanes of the type [B(C2H4SiCH3NCH3)3]n ([C14.4H31.4N4.1B1.0Si3.0]n) using a liquid-phase impregnation (LPI) process. The derived [B(C2H4SiCH3NCH3)3]n-carbon composite was cross-linked under ammonia at 200 °C and then thermolyzed under nitrogen at 1000 °C to generate a SiBCN-carbon composite. The carbon template was subsequently removed through thermal treatment at 1000 °C in an ammonia atmosphere to generate ordered mesoporous structures. XRD and TEM analyses revealed that as a negative replica of the CMK-3 template, the obtained amorphous mesoporous material exhibited open, continuous, and ordered 2D hexagonal frameworks, whereas elemental analyses indicated the formation of materials with an empirical formula of Si3.0B1.0C4.2N3.5. The ordered mesoporous Si3.0B1.0C4.2N3.5 material displayed high surface area (600 m2 g-1), high pore volume (0.61 cm3 g-1), and narrow pore size distribution (around 3.4 nm) and exhibited a relatively good thermal stability in air through heat-treatment to 1400 °C.