The motion of an asymmetrical gyrostat, consisting of an asymmetrical carrier and an axisymmetric rotor, rotating about a fixed point under the action of the gravitational force, is studied in this paper. Deprit's canonical variables are introduced to describe the attitude motion of the gyrostat. We show that the motion is chaotic in the sense of Smale's horseshoe by using Melnikov's method developed by Holmes and Marsden. The effect of the rotor on the global motion of the gyrostat is also studied. It is found that, as the rotor speed increases, a chaotic motion will turn into a regular motion.