Raman spectra of few-layer phosphorene have been systematically studied using density functional theory calculations. We find that due to the interlayer van der Waals interactions, the low-frequency rigid layer Ag breathing mode and B1g shear mode can shift by as much as 45.1 cm(-1) and 38.5 cm(-1), respectively, as the layer numbers increase from 2L to 5L. In addition, a typical characteristic for the experimentally observable [Formula: see text] mode (∼460 cm(-1) in bulk) is identified. Interestingly, this mode changes from coupled in-plane and out-of-plane vibrations in single layer to pure in-plane vibrations in a few layers and the corresponding frequencies vary by as much as over 10 cm(-1). We argue that this Raman frequency variation might be used to experimentally characterize the thickness of this intriguing 2D layered material.