A pneumatic tool includes a shell with a chamber defined therein, a first passage communicated with the chamber and a second passage communicated with the chamber. A rotor is installed in the chamber for rotation when driven by pressurized air flowing into the chamber through the first passage. A shaft extends to the exterior of the shell from the rotor. A trigger is installed on the shell. A control device is put in the first passage and connected with the trigger for controlling the first passage. A muffler is put in the second passage. The muffler includes a plurality of balls.