Vaterite mesocrystals with hexagonal morphology and uniform size have been successfully synthesized in the presence of a N-trimethylammonium derivative of hydroxyethyl cellulose via aggregation mediated crystallization using a simple gas-diffusion method. The uniform hexagonal plates display sharp facets and edges even though they are formed by the aggregation of nanocrystals. The results demonstrate that each vaterite plate can be explained as consisting of aggregates of nanoparticles that share the same three dimensional orientation. A mechanism for the formation of hexagonal vaterite mesocrystals made of primary nanoparticles and hexagonal units si also presented. An understanding of the mesoscale transformation process will be helpful in controlling the aggregation driven formation of complex higher order structured materials and will provide new insights into biomineralization mechanisms. For example the spines of sea urchins can be discussed within the framework of the mesocrystal concept. This study could provide an additional tool for designing advanced materials and could be used for the synthesis of more complex crystalline three-dimensional structures.