It is widely believed that Amontons’ law of friction holds well in various systems. Here we show based on a numerical calculation and analytical theory that Amontons’ law breaks in viscoelastic materials under certain condition due to the precursor slip prior to the bulk sliding. It is also shown that instead of Amontons’ law new friction law holds, which states that the static friction coefficient decreases as a power functionof load with the power −1/3. The new friction law is verified by the experiment employing polymethyl-methacrylate. The relation between the critical length of the precursor and the static friction coefficient predicted by numerical calculation and analytical theory is also verified.