An improved 'chemical blowing' route presuming atmospheric-pressure pre-treatment and moderate heating rate of designated precursors was developed to synthesize ultra-thin boron nitride (BN) nanosheets with high yield and large lateral dimensions. The yield reached as high as 40 wt% with respect to raw materials (ammonia borane). The strong oxygen-related ultraviolet luminescence together with a blue emission of these BN nanosheets was then documented and analyzed. This implies potential applications in solid-state lighting, ultraviolet lasing and full-color luminescence. Mechanical strength of different polymeric composites with a small fraction of BN nanosheet fillers was dramatically increased by tens of per cent, while high transparency of composite materials was still maintained in the visible optical range. The increased yield and reduced cost of BN nanosheets should promote their wide practical applications in various composites.