We report on Boron diffusion and subsequent crystallization of Co40Fe40B20 (CoFeB) thin films on SiO2/Si(001) substrate using pulsed laser deposition. Secondary ion mass spectroscopy reveals Boron diffusion at the interface in both amorphous and crystalline phase of CoFeB. High-resolution transmission electron microscopy reveals a small fraction of nano-crystallites embedded in the amorphous matrix of CoFeB. However, annealing at 400°C results in crystallization of CoFe with bcc structure along (110) orientation. As-deposited films are non-metallic in nature with the coercivity (Hc) of 5Oe while the films annealed at 400°C are metallic with a Hc of 135Oe.