When a naturally fractured carbonate formation is treated with acid at pressures below the fracturing pressure, the acidizing process will likely be different from either a matrix treatment or an acid fracturing treatment. Our previous experimental study shows three kinds of acid etching patterns after acidizing naturally fractured carbonates, and it also illustrates their relationships with the fracture properties and acidizing conditions. Based on the experimental observations, a mathematical model of acidizing naturally fractured carbonates has been developed. The model includes bulk solution transport, acid transport and reaction, and the change of fracture width by acid dissolution. A new approach was used to treat leakoff acid and acid-rock reactions at the fracture walls. The new acidizing model for numerically generated rough-surfaced fractures predicts the same kinds of acid etching patterns and the same relationships between acid etching patterns and the fracture width, surface roughness, and leakoff rate as observed in experiments.