Cesium lead halide perovskite nanocrystals (NCs) have unique optical properties such as high color purity and high photoluminescence (PL) efficiency. However, the external quantum efficiency (EQE) of the corresponding light-emitting diodes (LEDs) is low, primarily as a result of the NC surface defects. Here, we report a method to reduce the surface defects by capping CsPbI3 NCs with PbS. This passivation significantly enhanced the PL efficiency, reduced the Stokes shift, narrowed the PL bandwidth, and increased the stability of CsPbI3 NCs. At the same time, CsPbI3 NC films switched from n-type behavior to nearly ambipolar by PbS capping, which allowed us to fabricate electroluminescence LEDs using p-i-n structures. The thus-fabricated LEDs exhibited dramatically improved storage and operation stability, and an EQE of 11.8%. These results suggest that, with a suitable surface passivation strategy, the perovskite NCs are promising for next-generation LED and display applications.