As a candidate material for plasma facing material in future fusion reactor, tungsten (W) fiber reinforced tungsten (W_f/W) composite has been recently developed. The crack resistance of W_f/W is proven to be significantly higher compared to normal tungsten. However, the W-fibers used always become embrittlement during the powder metallurgy (PM) processes. In order to understand this significant issue, in this work, a series of W_f/W composites have been prepared. Microstructural and mechanical studies revealed that microstructural and mechanical studies revealed that the nanosized carbides in the grains and the carbide-layer on the grain boundaries are formed during PM processes. Especially, the carbide-layer on the grain boundaries can cause the brittle fracture of those W-fibers affected. Meanwhile, W-foil protection of the green body during the sintering process can reduce the carbon contamination effect and allows to preserve the ductility of the tungsten fibers used.