Abstract: The development of the mathematic model of critical aspect ratio of short fiber composites was briefly introduced. Based on the characters of structure stress of short fiber/rubber composites and Cox shear lag, a new model of critical aspect ratio used for short fiber/rubber composites has been founded and then discussed in detail. The results showed that the new model can better suit the characters of short fiber/rubber composite; compared to other models, the new model can reflect totally the relations between the structures of the short fiber composite and its critical aspect ratio.