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This paper reports on the experimental and theoretical studies of a light-driven micromotor, which is a “light mill” that rotates by absorbing photon energy. This light mill has four curved blades to form an axially asymmetric geometry. Upon lateral irradiation, the shape of the light mill induces an asymmetric photon heating to the surrounding gas molecules, leading to a gas convection that forces the light mill to rotate. The light mill was applied to actuate a scanning mirror for a laser beam. Using a Direct Simulation Monte Carlo (DSMC) model, we investigated the working principle behind the operation of the light mill. The DSMC simulation yielded results consistent to our experimental data. The simulation results were used to explain the heat-induced light-mill rotation, in which the mean free path of the surrounding gas takes an important role.