Electrically insulating boron nitride (BN) nanosheets possess similar thermal conductivity while superior thermal and chemical stabilities comparing with electrically conductive graphenes. Currently the productions and applications of BN nanosheets are rather limited due to the complexity of the BN binary compound growth, opposite to massive graphene production. Here we developed an original strategy "biomass-directed on-site synthesis" toward mass-production of high-crystal-quality BN nanosheets. The strikingly effective, reliable, and high-throughput (dozens of gram) synthesis is directed by diverse biomass sources through the carbothermal reduction of gaseous boron oxide species. Produced BN nanosheets are single crystalline, laterally large and atomically thin. Additionally, they assemble themselves to the same macroscopical shapes peculiar to original biomasses. The nanosheets are further utilized for making thermoconductive and electrically insulating epoxy/BN composites with a 14-fold increase in thermal conductivity, which are envisaged to be particularly valuable for future high-performance electronic packaging materials.