A new concept is proposed to explain the formation of spherical boron nitride (BN) nanoparticles synthesized by the chemical vapor deposition (CVD) reaction of trimethoxyborane (B(OMe)3) with ammonia. The intermediate phases formed during the CVD under different reaction conditions are analyzed by X-ray diffraction, electron microscopy, thermogravimetry, and spectroscopy techniques. The transition mechanism from an intermediate B(OMe)3–xH3–xN (x < 2) phase having single BN bonds to the BN nanoparticles is elucidated. This particularly emphasizes the CVD temperature effect governing the conversion of the NH···OB hydrogen bonds in (OMe)3B · NH3 into the NB bonds in B(OMe)3–xH3–xN. The spherical morphology strongly depends on the remnant impurity oxygen formed upon Me2O group elimination in the intermediate. Two types of spherical BN nanoparticles primarily attractive for immediate commercialization (with C and H impurities at a level less than 1 wt %) are synthesized by the adjustment of experimental parameters: high oxygen-containing (∼6.3 wt %) BN spheres with a diameter of ∼90 nm and a specific surface area of 26.8 m2 g−1; and low oxygen-containing (<1 wt %) BN spheres with a diameter of ∼30 nm and a surface area of 52.7 m2 g−1. Finally, the regarded synthetic techniques are fully optimized in the present work.