Slope revegetation by seeding works with herbaceous plants is widely employed to prevent road embankment from erosion damage. The Percentage of Vegetational Cover (PVC) is used as an indicator for the quality evaluation of the seeding works. However, the PVC is usually measured manually through visual inspection, and the measurement results of the same subject might not be constant. In the present study, RGB image analysis is applied to objectively measure the PVC to overcome this situation. Then, laboratory flume experiments are conducted to investigate the capability of the PVC obtained by RGB image analysis to predict the decrease of sediment erosion against concentrated flow. Two herbaceous plant species, Centipede Grass and White Clover, are employed in the experiments. The findings of the experiments are as follows: Declining trends of the sediment erosion are observed as the PVC increases or plant growth period becomes longer, and the trends are more correlated with the PVC with RGB image analysis than with plant growth period. The results indicate that RGB image analysis can be a useful and inexpensive tool for inspection of the seeding works with herbaceous plants.