Yttrium silicates oxidation resistance coatings were deposited on SiC-C/C composites surface by a hydrothermal electrophoretic deposition process. The influence of deposition voltage on the yttrium silicates coatings and the deposition amount of coatings were particularly investigated. Results show that with the increase of deposition voltage, the deposition amount of coatings increase, the density and uniformity of the coating surface is improved. When the deposition voltage reaches 210V, dense yttrium silicates coatings are achieved. While increase the deposition voltage continually, the uniformity of coatings decrease. When the deposition voltage arrives to 240V, high deposition voltage will result in cracked coatings. There is a parabola relationship between the deposition amount of coatings and time.