The magnetic anisotropy of a thulium orthoferrite single crystal was investigated by means of an automatically recording torque-magnetometer in the range between 77 and 300°K. It was found that at room temperature the direction of weak magnetization, σ0, associated with an antiferromagnetic canted spin arrangement is parallel to the c axis of orthorhombic structure, and that at about 100°K a magnetic transition occurs, where the direction of σ0 changes from the c axis to the a axis. The transition temperature obtained is the highest among those reported before on other orthoferrites. The torque curves obtained above the transition temperature are similar to those by Sherwood et al. and can be explained by their model. The value of σ0 is equal to 0.06 μB/mole at room temperature. At 77°K below the transition temperature, however, inexplicable torque curves were obtained in the b plane. The curves showed a small jump at the c axis superimposed on a 2θ curve with large amplitude. The jump amplitude is almost independent of the applied field in the range of 8500 to 3000 oe. This behavior cannot be explained by Sherwood's model, but it will give information on the interaction between thulium and ferric ions.