We investigate the valley related carrier dynamics in monolayer MoS<sub>2</sub> using helicity resolved non-degenerate ultrafast
pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer
MoS<sub>2</sub> shows remarkable transient reflection signals, in stark contrast to bilayer and bulk MoS<sub>2</sub> due to the enhancement of
many-body effect at reduced dimensionality. The helicity resolved ultrafast time-resolved result shows that the valley
polarization is preserved for only several ps before scattering process makes it undistinguishable. We suggest that the
dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the
exfoliated MoS<sub>2</sub> samples.