  In this Letter, we present Interface Region Imaging Spectrograph Fe XXI
1354.08 A forbidden line emission of two magnetic flux ropes (MFRs) that caused
two fast coronal mass ejections with velocities of $\ge$1000 km s$^{-1}$ and
strong flares (X1.6 and M6.5) on 2014 September 10 and 2015 June 22,
respectively. The EUV images at the 131 A and 94 A passbands provided by the
Atmospheric Imaging Assembly on board Solar Dynamics Observatory reveal that
both MFRs initially appear as suspended hot channel-like structures.
Interestingly, part of the MFRs is also visible in the Fe XXI 1354.08 forbidden
line, even prior to the eruption, e.g., for the SOL2014-09-10 event. However,
the line emission is very weak and that only appears at a few locations but not
the whole structure of the MFRs. This implies that the MFRs could be comprised
of different threads with different temperatures and densities, based on the
fact that the formation of the Fe XXI forbidden line requires a critical
temperature ($\sim$11.5 MK) and density. Moreover, the line shows a non-thermal
broadening and a blueshift in the early phase. It suggests that magnetic
reconnection at that time has initiated; it not only heats the MFR and, at the
same time, produces a non-thermal broadening of the Fe XXI line but also
produces the poloidal flux, leading to the ascending of the MFRs.
