Cr-B thin films were synthesized by an r.f. plasma-assisted magnetron sputtering method using pure CrB2 target on quartz glass, glass ceramic, NaCl single crystal and silicon wafer substrates. After the sputtering process, the as-deposited Cr-B thin films were annealed at 773, 973 and 1173 K for 5 h. The influence of annealing temperature on the structure and properties of Cr-B thin films was investigated, Although the films were deposited at low temperature (T-S/T-M < 0.1) in this study, X-ray diffraction and transmission electron microscopy observation results indicate that fine crystalline grains are still formed in these films. With increasing annealing temperature, the grain size increased gradually from 10 to 30 nm. The compressive stress in the Cr-B films is rather small due to the long target-substrate distance of this apparatus which reduces the atomic peening effect. The hardness decreased scarcely with increasing annealing temperature from about 21 GPa for as deposited to about 19 GPa for 1173 K owing to the grain growth and relaxation of residual stress. The present study shows that Cr-B thin films can be used below 1200 K with stable structure and high hardness.