  We report experimental evidences on selective occupation of the degenerate
valleys in MoS2 monolayers by circularly polarized optical pumping. Over 30%
valley polarization has been observed at K and K' valley via the polarization
resolved luminescence spectra on pristine MoS2 monolayers. It demonstrates one
viable way to generate and detect valley polarization towards the conceptual
valleytronics applications with information carried by the valley index.
