We report full-color quantum-dot-based light-emitting diodes (QLEDs) with high efficiency and long-lifetime by employing high quantum-yield core/shell QDs with thick shells. The increased shell thickness improves the confinement of excitons in the QD cores, and helps to suppress Auger recombination and Förster resonant energy transfer among QDs. Along with optimizing the QD emitting layer thickness and hole transport materials, we achieved significant improvements in device performance as a result of increasing the QD shell thickness to above 5 nm. By using poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) as a HTL with a 38 nm thick QD layer, these QLEDs show maximum current efficiencies of 18.9 cd A-1, 53.4 cd A-1, and 2.94 cd A-1, and peak external quantum efficiencies (EQEs) of 10.2%, 15.4%, and 15.6% for red, green, and blue QLEDs, respectively, all of which are well maintained over a wide range of luminances from 102 to 104 cd m-2. To the best of our knowledge, this is the first report of blue QLEDs with ηEQE > 15%. Most importantly, these devices also possess long lifetimes with T70 (the time at which the brightness is reduced to 70% of its initial value) of 117 h (red, with an initial luminance of 8000 cd m-2), 84 h (green, 6000 cd m-2) and 47 h (blue, 420 cd m-2). With further optimization of QD processing and device structures, these LEDs based on thick-shell QDs show great promise for use in next-generation full-color displays and lighting devices.