We use the graphene as a passivation layer to protect the black phosphorus from chemical reaction with air. Flakes of the black phosphorus crystal are covered by N = 1, 2, and 3 layers of CVD graphene and the Raman phonon modes are measured after the samples are exposed to air. For the double and triple layer passivation (N = 2,3) the Raman phonon peaks are maintained as long as similar to 100 h (4 days) before the phonon intensity is reduced to 90% of the initial strength, I/I-0 = 0.9, demonstrating that the black phosphorus is strongly protected in contrast to the rapid degradation of the bare and single-layer covered black phosphorus. The long term stability achieved by multi-layer graphene passivation makes it possible to perform various electronic and optical experiments aimed for device application of this direct-gap van der Waals semiconductor with high electronic and thermal mobility under ambient environment. (C) 2015 Elsevier B.V. All rights reserved.