Abstract: The use of dispersants is a potentially effective response method for subsurface oil release, but few researches have been conducted on the quantitative evaluation of dispersant effectiveness in the subsurface. In this study, a test facility with the dimensions of 1 m×1 m×2 m (length×width×height) was used for conducting subsurface release experiments of Penglai 19-3 crude oil with dispersant. The oil droplet size distribution (volume%) and volume median diameter d₅₀ were chosen as the index to evaluate the influence of the amount of dispersant on the droplet breakup. The results showed that the dispersion of subsurface oil release was more significant with dispersant than without dispersant. The highest point of volume ratio shifted to the direction of small droplets, and the volume fraction of bigger droplets decreased. More dispersant was used, more the volume fraction of bigger droplets decreased, and total disappeared eventually. The volume median diameter d₅₀ also decreased with increasing dispersant amount.