The species in the gas phase of diborane chemical vapor deposition were examined on the basis of an ab initio molecular orbital calculation and in-situ mass spectroscopic analysis. Mass spectroscopic analysis revealed that the concentrations of B3Hx (x = 0-3) were high at the optimum condition for the film deposition. They were considered to be the main precursors of the icosahedral boron films. The can be produced by gas phase reactions of BHx and B2Hx. Stable structures and relative stabilities of B3Hx (n = 0-3), B2Hx (x = 0-4), and BHx (x = 1-3) were determined by using an ab initio molecular orbital method. In the case of B3Nx (x = 0-3), triangular forms are more stable than linear forms.