Ultrafine WC-10wt.%Co cemented carbides powders were synthesized by direct carburization. W-Co composite powders and carbon black powders were mixed by wet ball milling and dried. The mixed powders were heated to 800 $^{circ}C$ with heating rate of 8.2$^{circ}C$/min and held for various times in flowing $H_2$. For carbon addition of 140%, the carburization was completed by heating at 80$0^{circ}C$ for 4 hours. The carburization time decreased with increasing amount of carbon and carburization was completed by heating at 800 $^{circ}C$ for 2 hours with carbon addition of 150%. WC-10 wt%Co cemented carbides powders fabricated by direct carburization have nanoscale WC($/leqq$100 nm) size.