To improve the oral bioavailability of poorly water-soluble cyclosporin A (CyA), polymeric micelles based on monomethoxy poly(ethylene glycol)-b-poly(d,l-lactic acid) (mPEG-PLA) were prepared. In vitro release test showed that the cumulative release percentage, about 85%, of CyA from polymeric micelles within 24 h was comparable to that from Sandimmun Neoral, the currently available oral formulation of CyA. A relative oral bioavailability of 137% in rats compared with Sandimmun Neoral was demonstrated for CyA-loaded polymeric micelles. The other aim of the current work was to study the transport mechanism of mPEG-PLA micelles across the intestinal barrier. It was found that polymeric micelles could significantly increase the permeability of CyA across Caco-2 monolayers without significantly affecting transepithelial electrical resistance values, and the apparent permeation coefficient (P(app)) of CyA was significantly higher in the AP-BL direction compared to that in the BL-AP direction, suggesting that polymeric micelles might undergo an active AP to BL transport that probably involved endocytosis which was confirmed by confocal microscope observation. The permeation of CyA through Caco-2 monolayers showed that the P(app) was significantly increased when CyA was formulated with the copolymer below its critical association concentration (CAC) and no significant difference was found above its CAC, implying that mPEG-PLA monomers affected the intestinal P-gp efflux pumps. Therefore, the mPEG-PLA micelles seemed to be a good candidate for oral delivery of poorly soluble drugs.