The reactions of NH3BH2R (R = Me, Ph and Cl) with THFBH3 have been investigated and it was found that different substituents on the B atom help to proceed the reactions in different ways. The expected doubly‐bridged B‐substituted aminodiborane products, similar to aminodiborane (ADB, BH2(µ‐H)(µ‐NH2)BH2) via the reaction of ammonia borane (AB, NH3BH3) and tetrahydrofuran borane (THFBH3), are not obtained. Two competitive reactions occurred with the change of R = Me or Ph. When R is a Me group, an “open“ version of B‐substituted µ‐aminodiborane, THFBH(Me)(µ‐NH2)BH3, is formed as a major product; when R is a Ph group, AB and THFBH2Ph are formed as main products via the intermolecular NH3–THF exchange reaction. However, if R is Cl, then NH3BH2Cl reacts with THFBH3 through reversible intermolecular Cl–H exchange mechanism. Furthermore, DFT calculations are performed to elucidate the formation mechanism of THFBH(Me)(µ‐NH2)BH3 via the reaction of NH3BH2Me and THFBH3 as well as the exchange mechanism of Cl–H in the reaction of NH3BH2Cl and THFBH3.