In order to improve the hydrogen storage properties of LiBH_4-MgH_2 composite, two different kinds of Nb-based catalysts, NbC and NbF_5, were added to LiBH_4-MgH_2 composite by ball milling, and the effect of catalysts on hydrogen storage properties of the modified LiBH_4-MgH_2 system was investigated. The experimental results show that LiBH_4-MgH_2 composite is a two-step dehydrogenation process, and Nb-based compounds can remarkably enhance its dehydrogenation kinetics. For the composite without addition of catalysts, the starting decomposition temperature for the first dehydrogenation step is around 320℃, and there is a long period of incubation time(around 220 min) for the occurrence of the second decomposition step even at high temperature of 450℃. It needs more than 10 h to complete the decomposition process and release around 9 wt% H_2. After addition of 5 mol% NbF_5, the starting decomposition temperature for the first dehydrogenation step is around 150℃, there is no incubation time for the second decomposition step, and it takes around 40 min to complete the second step and reaches a total dehydrogenation capacity of 9.5 wt%. NbF_5 has better catalytic effect than NbC. Based on the hydrogenation/dehydrogenation behaviors and structural variation, the mechanism of catalytic effect was discussed. In order to improve the hydrogen storage properties of LiBH_4-MgH_2 composite, two different kinds of Nb-based catalysts, NbC and NbF_5, were added to LiBH_4-MgH_2 composite by ball milling, and the effect of catalysts on hydrogen storage properties of the modified The experimental results show that LiBH_4-MgH_2 composite is a two-step dehydrogenation process, and Nb-based compounds can remarkably enhance its dehydrogenation kinetics. For the composite without addition of catalysts, the starting decomposition temperature for the first dehydrogenation step is around 320 ° C., and there is a long period of incubation time (around 220 min) for the occurrence of the second decomposition step even at high temperature of 450 ° C. It needs more than 10 h to complete the decomposition process and release around 9 wt% H 2. After addition of 5 mol% NbF 5, the starting decomposition temperature for the first dehydrogenation step is around 150 ° C., there is no incu bation time for the second decomposition step, and it takes around 40 min to complete the second step and reaches a total dehydrogenation capacity of 9.5 wt%. NbF_5 has better catalytic effect than NbC. Based on the hydrogenation / dehydrogenation behaviors and structural variation, the mechanism of catalytic effect was discussed.