Ammonia borane (AB), with remarkable advantages of the top hydrogen content (19.6wt.%) and easily controlled dehydrogenation, has been being regarded as one of the most competitive hydrogen storage materials. The serious shortage of feasible and low-cost synthesizing technique that is applicable to AB production, to date, largely restricts its massive application. We here report a novel synthesizing process, or liquid phase oxidizing route, to prepare highly purified AB crystals using Cu (II)-NH3 complex as both oxidizer and nitrogen source reactant. The effects of both solvents and temperatures on AB yield and purity were separately studied in order for determining the optimal process parameters. The related phase constitution and morphology of the as-prepared AB crystals were analyzed. The purity measuring of the AB crystals was systematically investigated by both indirect and direct analysis methods. The results show that the AB product with yield of more than 88% and purity of higher than 98% can be synthesized by means of the liquid phase oxidizing strategy, and ether-type solvents (eg, straight-chain ether or ringlike ether) are more recommendable option.