Technique of a newly developed in-situ electron-spin-resonance (ESR) is introduced for understandings of microscopic mechanism of dangling-bond creation and annihilation in Si oxidation processes as well as in deposition and plasma treatments of hydrogenated amorphous silicon (a-Si: H). 1) Dynamic changes of surface states during the Si oxidation process have been observed; ad-atom dangling bonds in Si(111)7×7 structure, termination of ad-atom dangling bonds due to oxidation, and creation of interface dangling bonds between Si and SiO₂ thus formed. 2) Dynamic changes of the Si dangling-bond signal intensity were observed during and after the a-Si: H deposition, which confirms the existence of a subsurface region with a quite high spin density (10¹³cm⁻²) only during the deposition process. 3) During hydrogen plasma treatments on a-Si: H hydrogen atoms create dangling bonds, rather than terminate them. These dangling bonds are not confined to the film surface but are spatially distributing to the deeper layers from the top-surface (around 100 nm), whose depth decreases with the increase in the treatment temperature.