Abstract: When acid treatment of a naturally fractured carbonate formation is below fracture pressure, the acidizing process is neither different from common matrix treatment, nor from acid fracturing. As for this reason, a mathematical model of acidizing naturally fractured carbonates has been developed. The model includes mass conservation of acid liquid, as well as acid transmission and reacting, and the fracture width variance along with the change of acid dissolution. A new approach is also applied in acid leak - off and acid - rock reactions of fractured walls. With this new acidizing model for numerically generated rough - surfaced fractures, it can predict acid etching patterns and its relations with the fracture width, surface roughness, and leak-off rate. Beyond that, it also can predict the variance of pressure, acid concentration and fracture width along with the changing of time and space.