Provided are a rubber latex used as a substrate for an impact modifier, a preparation method thereof, and an impact modifier prepared using the rubber latex. The rubber latex includes a rubber monomer as a main component and has a decreasing gel content from a core to a shell (s). The preparation method includes polymerizing a core followed by Polymerization of a shell (s) has a lower gel content that the core. The impact modifier is prepared by common graft polymerization using The rubber latex as a substrate. The rubber latex has a high gel content core and a low gel content shell (s), and thus, is free from problems involved In low or high gel content rubber particles. The rubber latex can be used As a substrate for a high efficiency impact modifier with high rubber Content and enhanced impact strength and processability.