In this study, the effect of the sequential He and NH3 plasma treatments on a chemical vapor deposition SiOC:H low-k dielectric is evaluated in the wide range of experimental conditions. Results show that the active NH3 plasma radicals penetrate the porous low-k bulk and remove the hydrophobic Si-CH3 groups, which leads to hydrophilization and results in the degradation of dielectric properties. The implementation of He plasma pretreatment reduces the damage imposed by the NH3 plasma by a stimulation of the surface recombination of active radicals from NH3 plasma. He plasma causes a surface modification of 10-20 nm presumably due to the energy transfer from the extreme UV photons and the 2(1)S He metastable atoms to the low-k structure. The plasma damage reduction is proportional to He plasma density and the treatment time. The mechanism of plasma damage reduction is explained on the basis of the Knudsen diffusion mechanism and random walk theory.