The thermal decomposition pathway of sodium amidoborane (NaAB; NaNH2BH3) has been investigated in detail by using solid state NMR spectroscopy. 23Na MAS/3QMAS NMR spectra suggested that NaH and an amorphous Na–N–B–H phase started to be formed as decomposition products even at 79 °C, although NaAB was prepared from NaH and NH3BH3 by ball milling at room temperature. Based on the quantitative analyses of the 23Na MAS spectra, we proposed a decomposition reaction to 200 °C to be NaNH2BH3 → Na0.5NBH0.5 + 0.5NaH + 2.0H2. The hypothetical phase Na0.5NBH0.5 is amorphous, where the basic molecular unit of the original NaAB is polymerized into a [–BN–]n network structure. It was also found that the diammoniate of diborane (DADB) and polyaminoborane (PAB) were not formed during the decomposition of NaAB, which are both key compounds on the pyrolysis of ammonia borane (AB).