The eubacterial chaperonins GroEL and GroES are essential chaperones and primarily assist protein folding in the cell. Although the molecular mechanism of the GroEL system has been examined previously, the mechanism by which GroEL and GroES assist folding of nascent polypeptides during translation is still poorly understood. We previously demonstrated a co-translational involvement of the Escherichia coli GroEL in folding of newly synthesized polypeptides using a reconstituted cell-free translation system (Ying, B. W. et al., J. Biol. Chem. 280, 12035-12040, 2005). Employing the same system here, we further characterized the mechanism by which GroEL assists folding of translated proteins via encapsulation into the GroEL-GroES cavity. The stable co-translational association between GroEL and the newly synthesized polypeptide is dependent on the length of the nascent chain. Furthermore, GroES is capable of interacting with the [GroEL-nascent-peptide-ribosome] complex, and experiments using a single-ring variant of GroEL clearly indicate that GroES association occurs only at the trans-ring, not the cis-ring, of GroEL. GroEL holds the nascent chain on the ribosome in a polypeptide length-dependent manner, and post-translationally encapsulates the polypeptide using the GroES cap to accomplish the chaperonin-mediated folding process.