The mammalian HRD1-SEL1L complex provides a scaffold for endoplasmic reticulum (ER)-associated degradation (ERAD), thereby connecting luminal substrates for ubiquitination at the cytoplasmic surface after their retrotranslocation through the endoplasmic reticulum membrane. In this study the stability of the mammalian HRD1-SEL1L complex was assessed by performing siRNA-mediated knockdown of each of its components. Although endogenous SEL1L is a long-lived protein, the half-life of SEL1L was greatly reduced when HRD1 is silenced. Conversely, transiently expressed SEL1L was rapidly degraded but was stabilized when HRD1 was coexpressed. This was in contrast to the yeast Hrd1p-Hrd3p, where Hrd1p is destabilized by the depletion of Hrd3p, the SEL1L homologue. Endogenous HRD1-SEL1L formed a large ERAD complex (Complex I) associating with numerous ERAD components including ERAD lectin OS-9, membrane-spanning Derlin-1/2, VIMP, and Herp, whereas transiently expressed HRD1-SEL1L formed a smaller complex (Complex II) that was associated with OS-9 but not with Derlin-1/2, VIMP, or Herp. Despite its lack of stable association with the latter components, Complex II supported the retrotranslocation and degradation of model ERAD substrates α1-antitrypsin null Hong-Kong (NHK) and its variant NHK-QQQ lacking the N-glycosylation sites. NHK-QQQ was rapidly degraded when SEL1L was transiently expressed, whereas the simultaneous transfection of HRD1 diminished that effect. SEL1L unassociated with HRD1 was degraded by the ubiquitin-proteasome pathway, which suggests the involvement of a ubiquitin-ligase other than HRD1 in the rapid degradation of both SEL1L and NHK-QQQ. These results indicate that the regulation of the stability and assembly of the HRD1-SEL1L complex is critical to optimize the degradation kinetics of ERAD substrates.