Precursor-derived Si-C-N ceramics after creep testing in air were characterized using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). XRD analysis showed that the crept Si-C-N ceramics were covered by an alpha-cristobalite layer. TEM observations revealed that the precipitated nanocrystallites in the crept Si-C-N ceramics were beta-SiC. Between alpha-cristobalite and crept Si-C-N ceramic, there was an intermediate zone in which Si2N2O nanocrystallites were distributed homogeneously. Moreover, Si2N2O nanocrystallites were often found covering the surface of nanosized gas channels in the crept Si-C-N ceramics, where no alpha-cristobalite phase was detected. Based on these observations, a two-step oxidation mechanism of Si-C-N ceramics during creep testing in air was proposed.