In the present study, B4C-BN nanocomposite powders were synthesized by using the sol-gel method. To investigate the effects of polyelectrolyte on phase content, particle size, and final morphology of synthesized powders different amounts of ammonium polycarboxylate were used as a gel dispersing agent and a nitrogen source. Highly crystalline, sub-micron/micron-sized boron carbide particles with varying morphologies including polyhedral-equiaxed, belt-like, needle-like, and complex-shaped hierarchical structures were produced from the polymeric gel containing glycerine, tartaric acid, and citric acid as carbon sources, and boric acid as boron source. With the addition of ammonium polycarboxylate as a polymeric gel network modifier, nanocomposite powders containing micron-sized polyhedral-equiaxed boron carbide particles and boron nitride nanoflakes were obtained. The results indicated that the particles dimensions, crystallinity, and B4C to BN phase ratio of the synthesized powders are directly related to the preliminary formation of borate-ammonium and/or amine complexes in the polymeric gel. The SEM inspections revealed that the size of boron carbide particles tends to increase from 2 μm up to 40 μm as a function of ammonium polycarboxylate content. It was also observed that the average size and thickness of boron nitride nanoflakes within the range of 80 nm to 3 μm and 10–150 nm, respectively. B4C/BN nanocomposite powders were synthesized with up to 32% BN content using a 43 wt% ammonium polycarboxylate additive.