Yang, Z.T. Hao, J.K. Liu, H. Liu, K.X. Quan, S.W. JACoW Publishing Geneva, Switzerland Mild, medium, and high temperature baking has been researched to obtain high-Q₀ SRF niobium cavities in past decades. It suggests that niobium has different properties when treated at different temperatures. We conducted various experiments on SRF-cavity-class niobium samples, and the systematic measurements included not only impurity analysis via TOF-SIMS, in-situ XPS, in-situ ESEM, HRTEM, but also superconductor measurements via in-situ ARPES. We also performed quantitative atomic simulation of the impurities in niobium bulks at zero temperature, and found interstitial carbon had similar trapping effect on interstitial hydrogen as interstitial nitrogen and oxygen did. We found the mildly increased interstitial carbons and oxygens during medium temperature baking not only suppressed the hydrogen accumulation and hydride precipitation during cooling down, but also reduced the electron mean free path to the optimal range which yielded declined BCS resistance. Therefore, the surface resistances of the cavities were reduced and the Q₀ values were improved correspondingly.