The structural and dielectric properties were investigated in the La2O3 added glass-ceramics based on complex niobates. With the addition of La2O3, the optimization of microstructure was observed which resulted in the improvement of breakdown strength for the glass-ceramics. Besides, the dielectric constant was drastically enhanced because of the doping effect of La3+ in the A-sites of both crystallographic structures. Due to the combined effects of both high breakdown strength and polarization difference, the maximum discharged energy density of 1.2J/cm3 was achieved in the niobate glass-ceramics with 2mol% La2O3, suggesting dielectric glass-ceramics of this composition could be the most attractive candidate for high-energy density capacitors.