We studied in detail the paramagnetic defect generated in the porous low-k SiOCH films, which is called the T-b center with the oxygen-carbon mixed back bonds. We baked the SiOCH films in vacuum from 600 to 1000 degrees C, which could correspond to temperature elevation at a local area in cases of UV or electron-beam curing and Joule heating in circuits. The amount of the T-b center increased abruptly around 775 degrees C by electron spin resonance spectroscopy, at which we observed 30% volume shrinkage by ellipsometry and network change from a cage link to a ring link by Fourier transform IR spectroscopy. These phenomena might be due to the crush of microholes wrapping Si-CH3 end groups. Therefore, we consider that the generation of the T-b center suggests the quality change of the porous low-k SiOCH films for the ultralarge-scale integration reliability.