Impact of the helium plasma exposure on the surface modification in tungsten and reduced activation ferritic/martensitic (RAFM) steel have been investigated on the linear plasma device PSI-2 assuming the condition of DEMO first wall. In tungsten, a nanoscale undulating surface structure, which has a periodic arrangement, is formed under low temperature conditions below fuzz nanostructure formation threshold ∼1000 K. Interval and direction of the undulation shows dependence on the crystal orientation. A large variation in surface level up to 200 nm has been observed among grains at a fluence of 3 × 10 26 He m−2 showing dependence of the surface erosion rate on the crystal orientation. The {100} plane in which the undulating surface structure is not formed shows the highest erosion rate. This significant erosion is due to the multistage sputtering through impurity. In RAFM steel, sponge-like nanostructure is developed and it grows with increasing helium fluence beyond 1 m. In the sponge-like nanostructure, a composition change from the base material is observed in which the tungsten ratio increases while the iron ratio decreases showing differences in sputtering ratio depending on the atomic mass.