Preparation of NbB2-Al2O3 in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions. Three different thermite mixtures, including Al-Nb2O5, Al-B2O3, and Al-Nb2O5-B2O3, were adopted and mixed with Nb and/or B powders to formulate the sample compacts. Experimental observations show that the thermite reaction enhances sustainability of the synthesis reaction. Among them, the powder compact containing the thermite mixture of Al and Nb2O5 exhibited the highest flame velocity and combustion temperature. The lowest reaction propagation rate and temperature were observed in the sample with Al and B2O3 as thermite reagents. Due to insufficient reaction time for the sample containing Al and Nb2O5, the end products were composed of large amounts of two intermediate borides Nb3B4 and NbB in addition to NbB2 and Al2O3. The SHS process involving the thermite reaction of Al with B2O3 was favorable to produce NbB2-Al2O3 composites, confirmed by the least Nb3B4 and NbB left in the end products.