It has been suggested that relativistic jets in quasars may contain a considerable amount of thermal matter .
In this paper , we explore the possibility that the K \alpha line from the thermal matter may appear at tens of keV due to a high Doppler blue-shift .
In the jet comoving frame , the energy density of photons originally emitted by the accretion disk and reflected off the broad line region clouds dominates over that of photons of other origin .
We discuss the photoionization states of the thermal matter and find that the irons elements are neutral .
The high metallicity in quasars enhances the possibility to detect the thermal matter in the relativistic jet in some radio-loud quasars .
A highly Doppler blue-shifted K \alpha line may be detected .
We make a prediction for 3C 273 , in which the K \alpha line luminosity might be of the order 3.0 \times 10 ^ { 44 } { erg~ { } s ^ { -1 } } with an equivalent width of 2.4 keV .
Such a line could be detected in a future mission .