Organic-inorganic hybrid perovskites have shown tremendous potential for optoelectronic applications. Ion migration within the crystal and across heterointerfaces, however, imposed severe problems with material degradation and performance loss in devices. Encapsulating hybrid perovskite with a thin physical barrier can be essential for suppressing the undesirable interfacial reactions without inhibiting the desirable transport of charge carriers. Here, we demonstrated that nanoscale, pinhole-free Al2O3 layer can be coated directly on the perovskite CH3NH3PbI3 using atomic layer deposition (ALD). The success can be attributed to a multitude of strategies including surface molecular modification and hybrid ALD processing combining the thermal and plasma-enhanced modes. The Al2O3 films provided remarkable protection to the underlying perovskite films, surviving by hours in solvents without noticeable decays in either structural or optical properties. The results advanced the understanding of applying ALD directly on hybrid perovskite and provided new opportunities to implement stable and high-performance devices based on the perovskites.