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The detection of the gamma-ray burst (GRB) X-ray emission line is importantfor studying the GRB physics and constraining the GRB redshift. Since theline-like feature in the GRB X-ray spectrum was first reported in 1999, severalworks on line searching have been published over the past two decades. Eventhough some observations on the X-ray line-like feature were performed, thesignificance remains controversial to date. In this paper, we utilize thedown-Comptonization mechanism and present the time evolution of the FeK$\alpha$ line emitted near the GRB central engine. The line intensitydecreases with the evolution time, and the time evolution depends on the theelectron density and the electron temperature. In addition, the initial linewith a larger broadening decreases less over time. For instance, when theemission line penetrates material with the an electron density above $10^{12}$cm$^{-3}$ at 1 keV, it generally becomes insignificant enough after 100 s forit not to be detected. The line-like profile deviates from the Gaussian form,and it finally changes to be similar to a blackbody shape at the time of thethermal equilibrium between the line photons and the surrounding material.

Non-detection of the Gamma-ray Burst X-ray Emission Line: The Down-Comptonization Effect