The diffraction efficiency of compression gratings in chirped-pulse amplification is very crucial to obtain terawatt femtosecond laser. Although the gratings are located in vacuum chamber with oil free, the gratings illuminated by laser beam are still subjected to carbon contamination in the vacuum condition, and results in degradation of diffraction efficiency. Moreover, additional energy deposition on the surface of grating will lead to irreversible changes of the grating structure or even catastrophic damage. A plasma cleaning technique is introduced to realize cleaning of carbon layer on the grating. The diffraction efficiency of grating is obviously improved after cleaning. It is demonstrated that the plasma cleaning is an effective on-line technique to repeatedly recover the high efficiency of compression gratings before damage occurs due to excessive carbon deposition.