Long BNNTs were synthesized using an optimized ball milling and annealing process. In a typical process, amorphous boron (B) powder was rst loaded into a stainless steel milling jar with steel balls (25.4 mm in diameter AISI 420) under NH3 atmosphere at 300 kPa. e weight ratio of the steel balls to B powder is 50:1. e ball milling process was performed for 50 h at a rotation speed of 300 rpm. is milling process introduces about 1.5 at% iron nanoparticles homogeneously to the B powder. e milled powder was then annealed at 1100°C in a tube furnace in an N2 ow of 100 mL/min. After annealing for 15 h, a large amount of long BNNTs was produced.