A powder mixture of lanthanum oxide (La2O3) and lanthanum fluoride (LaF3) was ground by a planetary ball-mill to investigate the mechanochemical reaction forming lanthanum oxyfluoride (LaOF) at room temperature. The grinding enables us to form LaOF monophase, and the reaction proceeds with an increase in grinding time, whereas the crystallite size of LaOF formed is about 15-20 nm irrespective of the grinding time. Other rare-earth oxyfluorides (R-OF, R = Pr, Nd, Sm, Gd) can be synthesized by grinding the constituent components (R2O3 and RF3).